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LIBRARY 

UNIVERSITY  Of 


EDUCPSYCH 


THE   ANIMAL   MIND 


THE  MACMILLAN  COMPANY 

NEW  YORK   •    BOSTON   •   CHICAGO  -   DALLAS 
ATLANTA  •    SAN    FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON  •    BOMBAY  •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


THE  ANIMAL  BEHAVIOR  SERIES.    VOLUME  II 


THE   ANIMAL   MIND 

A  Text-book  of  Comparative 
Psychology 


BY 


MARGARET  FLOY  WASHBURN,   PH.D. 

PROFESSOR  OF   PSYCHOLOGY  IN   VASSAR 
COLLEGE 


SECOND  EDITION 


Wefo  gorfc 

THE   MACMILLAN   COMPANY 
1917 

A  U  rights  rtstrved 


COPYRIGHT,  1908  AND  1917, 
Bv  THE  MACMILLAN  COMPANY. 

Set  up  and  electrotyped.     Published  February,  1908.     Reprinted] 
July,  1909;  August,  1913. 

Second  edition,  revised,  September,  1917. 


EDUC.- 

PSYCH. 
LIBRARY 


Add'l 


GIFT 


Norinoob 

J.  8.  Gushing  Co.  —  Berwick  <k  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


QJL  7?6 


EOUC.. 
PSYCH. 


PREFACE  TO  THE  FIRST 


EDITW 


THE  title  of  this  book  might  more  appropriately,  if  not  \ 
more  concisely,  have  been  "The  Animal  Mind  as  Deduced  j 
from  Experimental  Evidence."  For  the  facts  set  forth  in 
the  following  pages  are  very  largely  the  results  of  the  ex- 
perimental method  in  comparative  psychology.  Thus 
many  aspects  of  the  animal  mind,  to  the  investigation  of 
which  experiment  either  has  not  yet  been  applied  or  is  per- 
haps not  adapted,  are  left  wholly  unconsidered.  This  limi- 
tation of  the  scope  of  the  book  is  a  consequence  of  its  aim 
to  supply  what  I  have  felt  to  be  a  chief  need  of  compara- 
tive psychology  at  the  present  time.  Although  the  science 
is  still  in  its  formative  stage,  the  mass  of  experimental  ma- 
terial that  has  been  accumulating  from  the  researches  of 
physiologists  and  psychologists  is  already  great,  and  is  also 
for  the  most  part  inaccessible  to  the  ordinary  student,  being 
widely  scattered  and  to  a  considerable  extent  published  in 
journals  which  the  average  college  library  does  not  contain. 
While  we  have  books  on  animal  instincts  and  on  the  inter- 
pretation of  animal  behavior,  we  have  no  book  which  ade- 
quately presents  the  simple  facts. 

Probably  no  bibliography  seems  to  one  who  carefully 
examines  it  entirely  consistent  in  what  it  includes  and  what 
it  excludes.  Certainly  the  one  upon  which  this  book  is 
based  contains  inconsistencies.  The  design  has  been  to  ex- 
clude works  bearing  only  upon  general  physiology,  upon  the 
morphology  of  the  nervous  system  and  sense  organs,  or  upon 
the  nature  of  animal  instinct  as  such,  and  to  include  those 
which  bear  upon  the  topics  mentioned  in  the  chapter  head- 
ings. Within  these  limits,  the  collection  of  references  upon 

V 

'       543 


vi  Preface  to  the  First  Edition 

no  topic  is  as  full  as  would  be  necessary  for  the  bibliography 
of  a  special  research  upon  that  topic.  Doubtless  there  are 
omissions  for  which  no  excuse  can  be  found.  In  one  or  two 
cases,  where  the  literature  upon  a  single  point  is  very  large, 
as  for  example,  in  the  case  of  the  function  of  the  semicircular 
canals,  only  a  few  of  the  more  important  references  have 
been  given. 

One  further  comment  may  be  made.  The  book  through- 
out deals  with  comparative  rather  than  with  genetic  psy- 
chology. 

I  gratefully  acknowledge  help  from  a  number  of  sources. 
To  Professor  Titchener  I  owe,  not  only  my  share  of  that 
genuine  psychological  spirit  which  he  so  successfully  imparts 
to  his  pupils  according  to  their  ability,  but  various  helpful 
criticisms  upon  the  present  work,  about  half  of  which  he 
has  read  in  manuscript.  Dr.  Yerkes  has  given  me  much 
invaluable  aid  in  securing  access  to  material,  and  has  very 
kindly  permitted  me  to  see  the  proofs  of  his  book  on  "The 
Dancing  Mouse."  As  editor  of  the  series  he  has  reviewed 
my  manuscript  to  its  great  advantage.  Professors  Georges 
Bonn  and  George  H.  Parker  have  showed  especial  courtesy 
in  making  their  work  accessible  to  me.  Professor  Jennings 
has  kindly  allowed  the  use  of  a  number  of  illustrations  from 
his  book  on  "The  Behavior  of  the  Lower  Organisms."  My 
colleague  Professor  Aaron  L.  Tread  well  has  generously 
helped  me  in  ways  too  numerous  to  specify.  But  perhaps 
my  heaviest  single  obligation  is  to  Professor  I.  Madison 
Bentley,  who  has  read  the  manuscript  of  the  entire  book, 
and  whose  advice  and  criticism  have  been  of  the  utmost 

benefit  to  every  part  of  it. 

M.  F.  W. 

VASSAR  COLLEGE,  POUGHKEEPSIE,  N.Y. 
October  i,  1907. 


PREFACE  TO  THE   SECOND   EDITION 

THE  advance  of  comparative  psychology  during  the 
past  nine  years  has  been  remarkable.  In  preparing  a 
second  edition  of  this  book  I  have  tried  to  include  every 
newly  discovered  fact  of  the  first  importance,  but  the 
literature  is  now  so  extensive  that  in  order  to  keep  the 
bibliography  within  reasonable  limits,  I  have  had  to  exer- 
cise more  selection  than  I  did  in  preparing  the  bibliography 
for  the  first  edition.  For  like  reasons,  the  text  of  the  book 
does  not  enter  so  fully  into  detail  in  describing  the  results 
of  a  particular  investigation  as  was  possible  when  the 
material  at  hand  was  so  much  less  in  amount. 

More  than  half  the  book  has  been  completely  rewritten, 
including  the  chapters  on  Vision,  on  Spatially  Determined 
Reactions,  and  on  The  Modification  of  Conscious  Pro- 
cesses by  Individual  Experience.  I  hope  that  the  edition 
represents  an  advance  upon  its  predecessor,  not  only  by 
including  many  newly  ascertained  facts,  but  also  by  pre- 
senting its  subject  matter  in  more  logical  form. 

M.  F.  W. 

VASSAR  COLLEGE,  POUGHKEEPSIE,  N.Y., 
April,  1917. 


vii 


TABLE  OF   CONTENTS 

CHAPTER  I 

PAGES 

THE    DIFFICULTIES    AND    METHODS    OF    COMPARATIVE    PSY- 
CHOLOGY       . 1-26 

i.  Difficulties.     2.  Methods   of   Obtaining    Facts:    An- 
ecdote.   3.  Methods    of    Obtaining    Facts:     Experiment. 

4.  Methods    of    Obtaining    Facts:   the    Ideal    Method. 

5.  Methods  of  Interpreting  Facts. 

CHAPTER  II 

THE  EVIDENCE  OF  MIND 27-37 

6.  Inferring  Mind  from  Behavior.    7.  Inferring  Mind 
from  Structure. 


CHAPTER  III 

THE  MIND  OF  THE  SIMPLEST  ANIMALS 38-52 

8.  The  Structure  and  Behavior  of  Amoeba.  9.  The  Mind 
of  Amoeba. 

CHAPTER  IV 
SENSORY  DISCRIMINATION:  METHODS  OF  INVESTIGATION  .       .       53-62 

10.  Preliminary  Considerations,  n.  Structure  as  Evi- 
dence of  Discrimination.  12.  Behavior  as  Evidence  of 
Discrimination.  13.  Evidence  from  Structure  and  Behavior 
Combined.  14.  Evidence  for  Discrimination  of  Certain 
"Lower"  Sensation  Classes. 

CHAPTER  V 
SENSORY  DISCRIMINATION:  THE  CHEMICAL  SENSE     .        .        .      63-115 

15.  The  Chemical  Sense  in  Protozoa.  16.  The  Chemical 
Sense  in  Ccelenterates.  17.  The  Chemical  Sense  in  Flat- 

b 


Table  of  Contents 


worms.  1 8.  The  Chemical  Sense  in  Annelids.  19.  The 
Chemical  Sense  in  Mollusks.  20.  The  Chemical  Sense  in 
Echinoderms.  21.  The  Chemical  Sense  in  Crustacea. 
22.  The  Chemical  Sense  in  Arachnida.  23.  The  Chemical 
Sense  in  Insects.  24.  How  Ants  Find  Food.  25.  The 
Use  of  Smell  in  Path  Finding  by  Ants.  26.  How  Ants 
"Recognize"  Nest  Mates.  27.  How  Bees  Are  Attracted 
to  Flowers.  28.  How  Bees  Find  the  Hive.  29.  How 
Bees  "Recognize"  Nest  Mates.  30.  The  Chemical  Sense 
in  Vertebrates. 


CHAPTER  VI 

SENSORY  DISCRIMINATION:  HEARING          .        .       .       . 

31.  Hearing  in  Lower  Invertebrates.  32.  Hearing  in 
Crustacea.  33.  Hearing  in  Spiders.  34.  Hearing  in  In- 
sects. 35.  Hearing  in  Fishes.  36.  Hearing  in  Amphibia. 
37.  Hearing  in  Higher  Vertebrates. 


116-134 


CHAPTER  VII 

SENSORY  DISCRIMINATION:  VISION 

38.  Change  of  Light  Intensity  as  a  Stimulus.  39.  The 
Continuous  Action  of  Light:  Photokinesis.  40.  The 
Problem  of  Visual  Qualities :  Invertebrates.  41.  The  Prob- 
lem of  Visual  Qualities:  Amphioxus  and  Fish.  42.  The 
Problem  of  Visual  Qualities :  Reptiles  and  Amphibia.  43.  The 
Problem  of  Visual  Qualities :  Birds.  44.  The  Problem  of 
Visual  Qualities :  Mammals. 


CHAPTER  VIII 

SPATIALLY  DETERMINED  REACTIONS  AND  SPACE  PERCEPTION  . 
45.  Classes  of  Spatially  Determined  Reactions.  46.  Class 
I:  Reactions  to  a  Single  Localized  Stimulus.  47.  Class 
II:  Orienting  Reactions:  Possible  Modes  of  Producing 
Them.  48.  Orientation  to  Gravity:  Protozoa.  49.  Ori- 
entation to  Gravity:  Ccelenterates.  50.  Orientation  to 
Gravity:  Planarians.  51.  Orientation  to  Gravity:  An- 
nelids. 52.  Orientation  to  Gravity:  Mollusks.  53.  Ori- 
entation to  Gravity:  Echinoderms.  54.  Orientation  to 


172-214 


Table  of  Contents 


XI 


Gravity :  Crustacea.    55.  Orientation  to  Gravity :  Spiders 
and   Insects.      56.   Orientation   to   Gravity:    Vertebrates. 

57.  The    Psychic    Aspect    of    Orientation    to     Gravity. 

58.  Orientation    to    Light.     59.   Influences    Affecting    the 
Sense  of  Light  Orientations.    60.  The  Psychic  Aspect  of 
Orientation  to  Light.    61.  Mutual  Influence  of  Light  and 
Gravity  Orientations.    62.  Orientation  to  Other  Forces. 


CHAPTER  IX 
SPATIALLY  DETERMINED  REACTIONS  AND  SPACE  PERCEPTION 

63.  Class  III :  Reactions  to  a  Moving  Stimulus.  64.  Class 
IV:  Reaction  to  an  Image.  65.  Methods  of  Investigating 
the  Visual  Image.  66.  The  Visual  Perception  of  Size. 
67.  The  Visual  Perception  of  Form.  68.  The  Homing  of 
Animals  as  Evidence  of  Image  Vision.  69.  Class  IV: 
Reactions  Adapted  to  the  Distance  of  Objects.  70.  Some 
Theoretical  Considerations. 


215-244 


CHAPTER  X 

THE  MODIFICATION  OF  CONSCIOUS  PROCESSES  BY  INDIVIDUAL 

EXPERIENCE 

71.  Modifications  Due  to  Essentially  Temporary  Physio- 
logical States:  (a)  Heightened  Reaction  as  the  Result  of 
Previous  Stimulation.  72.  Modification  Due  to  Essen- 
tially Temporary  Physiological  States:  (b)  Cessation  of 
Reaction  to  a  Repeated  Slight  Stimulus.  73.  Modifica- 
tions Due  to  Relatively  Permanent  Effects  of  Stimuli. 

74.  Learning  Involving  the  Dropping  Out  of  Movements. 

75.  The  Formation  of  Systems  of  Successive  Movements. 


245-285 


CHAPTER  XI 

THE  MODIFICATION  OF  CONSCIOUS  PROCESSES  BY  INDIVIDUAL 

EXPERIENCE 

76.  The  Recognition  of  Landmarks.  77.  The  Memory 
Idea.  78.  Conditions  Favoring  the  Development  of  Memory 
Ideas.  79.  Some  Alleged  Instances  of  Remarkable  Mental 
Powers  in  Animals.  80.  Certain  Influences  Affecting 
Learning. 


286-313 


xii  Table  of  Contents 

CHAPTER  XII 

PAGES 

SOME  ASPECTS  OF  ATTENTION 314-323 

81.  The  Interference  of  Stimuli.  82.  Methods  of  Secur- 
ing Prepotency  of  Vitally  Important  Stimuli.  83.  The 
Peculiar  Characteristics  of  Attention  as  a  Device  to  Secure 
Prepotency. 


THE  ANIMAL  MIND 


THE  ANIMAL   MIND 


CHAPTER  I 

THE  DIFFICULTIES  AND  METHODS  OF  COMPARATIVE 
PSYCHOLOGY 

§  i.  Difficulties 

THAT  the  mind  of  each  human  being  forms  a  region  inac- 
cessible to  all  save  its  possessor,  is  one  of  the  commonplaces 
of  reflection.  His  neighbor's  knowledge  of  each  person's 
mind  must  always  be  indirect,  a  matter  of  inference.  How 
wide  of  the  truth  this  inference  may  be,  even  under  the  most 
favorable  circumstances,  is  also  an  affair  of  everyday  ex- 
perience :  each  of  us  can  judge  his  fellow-men  only  on  the 
basis  of  his  own  thoughts  and  feelings  in  similar  circum- 
stances, and  the  individual  peculiarities  of  different  members 
of  the  human  species  are  of  necessity  very  imperfectly  com- 
prehended by  others.  The  science  of  human  psychology 
has  to  reckon  with  this  unbridgeable  gap  between  minds  as 
its  chief  difficulty.  The  psychologist  may  look  into  his  own 
mind  and  study  its  workings  with  impartial  insight,  yet  he 
can  never  be  sure  that  the  laws  which  he  derives  from  such 
a  study  are  not  distorted  by  some  personal  twist  or  bias.  For 
example,  it  has  been  suggested  that  the  philosopher  Hume 
was  influenced  by  his  tendency  toward  a  visual  type  of 
imagination  in  his  discussion  of  the  nature  of  ideas,  which 
to  him  were  evidently  visual  images.  As  is  well  known,  the 
experimental  method  in  psychology  has  aimed  to  minimize 


2  The  Animal  Mind 

the  danger  of  confusing  individual  peculiarities  with  general 
mental  laws.  In  a  psychological  experiment,  an  unbiassed! 
observer  is  asked  to  study  his  own  experience  under  certain 
definite  conditions,  and  to  put  it  into  words  so  that  the  ex- 
perimenter may  know  what  the  contents  of  another  mind  are 
like  in  the  circumstances.  Thus  language  is  the  essential 
apparatus  in  experimental  psychology;  language  with  all 
its  defects,  its  ambiguity,  its  substitution  of  crystallized  con- 
cepts for  the  protean  flux  of  actually  lived  experience,  its 
lack  of  terms  to  express  those  parts  of  experience  which  are 
of  small  practical  importance  in  everyday  life,  but  which 
may  be  of  the  highest  importance  to  mental  science.  Out- 
side of  the  psychological  laboratory  language  is  not  always 
the  best  guide  to  the  contents  of  other  minds,  because  it  is 
not  always  the  expression  of  a  genuine  wish  to  communicate 
thought.  "Actions  speak  louder  than  words,"  the  proverb 
says ;  but  when  words  are  backed  by  good  faith  they  furnish 
by  far  the  safest  indication  of  the  thought  of  others. 
Whether,  however,  our  inferences  are  made  on  the  basis  of 
words  or  of  actions,  they  are  all  necessarily  made  on  the 
hypothesis  that  human  minds  are  built  on  the  same  pattern, 
that  what  a  given  word  or  action  would  mean  for  my 
mind,  this  it  means  also  for  my  neighbor's  mind. 

If  this  hypothesis  be  uncertain  when  applied  to  our  fellow 
human  beings,  it  fails  us  utterly  when  we  turn  to  the  lower 
animals.  If  my  neighbor's  mind  is  a  mystery  to  me,  how 
great  is  the  mystery  which  looks  out  of  the  eyes  of  a  dog,  and 
how  insoluble  the  problem  presented  by  the  mind  of  an 
invertebrate  animal,  an  ant  or  a  spider !  We  know  that  such 
minds  must  differ  from  ours  not  only  in  certain  individual 
peculiarities,  but  in  ways  at  whose  nature  we  can  only  guess. 
The  nervous  systems  of  many  animals  vary  widely  from  our 
own.  We  have,  perhaps,  too  little  knowledge  about  the 


Difficulties  and  Methods  3 

functions  of  our  own  to  conjecture  with  any  certainty  what 
difference  this  must  make  in  the  conscious  life  of  such  animals ; 
but  when  we  find  sense  organs,  such  as  the  compound  eyes 
of  insects  or  crustaceans,  constructed  on  a  plan  wholly  diverse 
from  that  of  ours ;  when  we  find  organs  apparently  sensory 
in  function,  but  so  unlike  our  own  that  we  cannot  tell  what 
purpose  they  serve,  —  we  are  baffied  in  our  attempt  to  con- 
struct the  mental  life  of  the  animals  possessing  them,  for 
lack  of  power  to  supply  the  sensation  elements  of  that  life. 
"It  is  not,"  said  Locke,  "in  the  power  of  the  most  exalted 
wit  or  enlarged  understanding,  by  any  quickness  or  variety  of 
thought,to  invent  or  frame  one  new  simple  idea  in  the  mind" 
(418,  Bk.  II,  ch.  2) ;  we  cannot  imagine  a  color  or  a  sound 
or  a  smell  that  we  have  never  experienced ;  how  much  less 
the  sensations  of  a  sense  radically  different  from  any  that  we 
possess !  Again,  a  bodily  structure  entirely  unlike  our  own 
must  create  a  background  of  organic  sensation  which  renders 
the  whole  mental  life  of  an  animal  foreign  and  unfamiliar  to 
us.  We  speak,  for  example,  of  an  "angry"  wasp.  Anger, 
in  our  own  experience,  is  largely  composed  of  sensations  of 
quickened  heart  beat,  of  altered  breathing,  of  muscular  ten- 
sion, of  increased  blood  pressure  in  the  head  and  face.  The 
circulation  of  a  wasp  is  fundamentally  different  from  that  of 
any  vertebrate.  The  wasp  does  not  breathe  through  lungs, 
it  wears  its  skeleton  on  the  outside,  and  it  has  the  muscles 
attached  to  the  inside  of  the  skeleton.  What  is  anger  like 
in  the  wasp's  consciousness?  We  can  form  no  adequate 
idea  of  it. 

To  this  fundamental  difficulty  of  the  dissimilarity  between 
animal  minds  and  ours  is  added,  of  course,  the  obstacle  that 
animals  have  no  language  in  which  to  describe  their  expe- 
rience to  us.  Where  this  unlikeness  is  greatest,  as  in  the 
case  of  invertebrate  animals,  language  would  be  of  little  use 


4  The  Animal  Mind 

since  we  could  not  interpret  it  from  our  experience ;  but  the 
higher  vertebrates  could  give  us  much  insight  into  their 
minds  if  they  could  only  speak.  We  are,  however,  restricted 
to  the  inferences  we  can  draw  from  movements  and  sounds 
that  are  made  for  the  most  part  without  the  intention  of 
communicating  anything  to  us.  One  happy  consequence  of 
this  fact,  which  to  a  slight  extent  balances  its  disadvantages, 
is  that  we  have  not  to  contend  with  self-consciousness  and 
posing,  which  often  invalidate  human  reports  of  intro- 
spection. 

From  these  general  considerations  we  can  understand 
something  of  the  special  difficulties  that  beset  the  path  of  the 
comparative  psychologist,  who  desires  to  know  the  contents 
of  minds  below  the  human  level.  Knowledge  regarding  the 
animal  mind,  like  knowledge  of  human  minds  other  than  our 
own,  must  come  by  way  of  inference  from  behavior.  Two 
fundamental  questions  then  confront  the  comparative  psy- 
chologist. First,  by  what  method  shall  he  find  out  how  an 
animal  behaves?  Second,  how  shall  he  interpret  the  con- 
scious aspect  of  that  behavior  ? 

§  2.   Methods  of  Obtaining  Facts:  The  Method  of  Anecdote 

The  reading  of  such  a  book  as  Romanes's  "Animal  Intel- 
ligence," or  of  the  letters  about  animal  behavior  in  the 
London  Spectator,  will  reveal  one  method  of  gathering  infor- 
mation about  what  animals  do.  This  has  been  termed  the 
Method  of  Anecdote.  It  consists  essentially  in  taking  the 
report  of  another  person  regarding  the  action  of  an  animal, 
observed  most  commonly  by  accident,  and  attracting  atten- 
tion because  of  its  unusual  character.  In  certain  cases  the 
observer  while  engaged  in  some  other  pursuit  happens  to 
notice  the  singular  behavior  of  an  animal,  and  at  his  leisure 


Difficulties  and  Methods  5 

writes  out  an  account  of  it.  In  others,  the  animal  is  a  pet, 
in  whose  high  intellectual  powers  its  master  takes  pride. 
It  is  safe  to  say  that  this  method  of  collecting  information 
always  labors  under  at  least  one,  and  frequently  under 
several,  of  the  following  disadvantages :  — 

1 .  The  observer  is  not  scientifically  trained  to  distinguish 
what  he  sees  from  what  he  infers. 

2.  He  is  not  intimately  acquainted  with  the  habits  of  the 
species  to  which  the  animal  belongs. 

3.  He  is  not  acquainted  with  the  past  experience  of  the 
individual  animal  concerned. 

4.  He  has  a  personal  affection  for  the  animal  concerned, 
and  a  desire  to  show  its  superior  intelligence. 

5.  He  has  the  desire,  common  to  all  humanity,  to  tell  a 
good  story. 

Some  of  these  tendencies  to  error  it  is  unnecessary  to  illus- 
trate. A  good  example  of  the  dangers  of  (2),  lack  of  ac- 
quaintance with  the  habits  of  the  species,  is  given  by  Mr. 
and  Mrs.  Peckham.  They  quote  the  following  anecdote 
reported  by  no  less  eminent  and  trained  an  observer  than 
Wundt.  "I  had  made  myself,"  says  that  psychologist, 
"as  a  boy,  a  fly- trap  like  a  pigeon  cote.  The  flies  were 
attracted  by  scattering  sugar  and  caught  as  soon  as  they 
had  entered  the  cage.  Behind  the  trap  was  a  second  box 
separated  from  it  by  a  sliding  door,  which  could  be  opened 
or  shut  at  pleasure.  In  this  I  had  put  a  large  garden 
spider.  Cage  and  box  were  provided  with  glass  windows 
on  the  top,  so  that  I  could  quite  well  observe  anything 
that  was  going  on  inside.  .  .  .  When  some  flies  had 
been  caught,  and  the  slide  was  drawn  out,  the  spider  of 
course  rushed  upon  her  prey  and  devoured  them.  .  .  . 
This  went  on  for  some  time.  The  spider  was  sometimes 
let  into  the  cage,  sometimes  confined  to  her  own  box.  But 


6  The  Animal  Mind 

one  day  I  made  a  notable  discovery.  During  an  absence 
the  slide  had  been  accidentally  left  open  for  some  little 
while.  When  I  came  to  shut  it,  I  found  that  there  was  an 
unusual  resistance.  As  I  looked  more  closely,  I  found  that 
the  spider  had  drawn  a  large  number  of  thick  threads 
directly  under  the  lifted  door,  and  that  these  were  prevent- 
ing my  closing  it.  .  .  . " 

"What  was  going  on  in  the  spider's  mind?"  Wundt  asks, 
and  points  out  that  it  is  unnecessary  to  assume  that  she 
understood  and  reasoned  out  the  mechanical  requirements 
of  the  situation.  The  whole  matter  can  be  explained,  he 
thinks,  in  a  simpler  way.  "I  imagine  that  as  the  days  went 
by  there  had  been  formed  in  the  mind  of  the  spider  a  deter- 
minate association  on  the  one  hand  between  free  entry  into 
the  cage  and  the  pleasurable  feeling  attending  satisfaction  of 
the  nutritive  impulse,  and  on  the  other  between  the  closed 
slide  and  the  unpleasant  feeling  of  hunger  and  inhibited  im- 
pulse. Now  in  her  free  life  the  spider  had  always  employed 
her  web  in  the  service  of  the  nutritive  impulse.  Associa- 
tions had  therefore  grown  up  between  the  definite  positions 
of  her  web  and  definite  peculiarities  of  the  objects  to  which 
it  was  attached,  as  well  as  changes  which  it  produced  in 
the  positions  of  certain  of  these  objects,  —  leaves,  small 
twigs,  etc.  The  impression  of  the  falling  slide,  that  is, 
called  up  by  association  the  idea  of  other  objects  similarly 
moved  which  had  been  held  in  their  places  by  threads 
properly  spun ;  and  finally  there  were  connected  with  this 
association  the  other  two  of  pleasure  and  raising,  unpleas- 
antness and  closing,  of  the  door"  (797,  pp.  351-352). 

The  Peckhams  remark  in  criticism  of  this  observation: 
"Had  Wundt  been  familiar  with  the  habits  of  spiders,  he 
would  have  known  that  whenever  they  are  confined  they 
walk  around  and  around  the  cage,  leaving  behind  them  lines 


Difficulties  and  Methods  7 

of  web.  Of  course  many  lines  passed  under  his  little  sliding 
door,  and  when  he  came  to  close  it  there  was  a  slight  resist- 
ance. These  are  the  facts.  His  inference  that  there  was 
even  a  remotest  intention  on  the  part  of  his  prisoner  to 
hinder  the  movement  of  the  door  is  entirely  gratuitous. 
Even  the  simpler  mental  states  that  are  supposed  to  have 
passed  through  the  mind  of  the  spider  were  the  products 
of  Wundt's  own  imagination"  (572,  p.  230).  The  fact  that 
the  anecdote  was  a  recollection  of  childhood,  so  that  it 
would  probably  be  impossible  to  bring  any  evidence  from 
the  character  of  the  web  or  other  circumstance  against 
the  suggestion  of  Mr.  and  Mrs.  Peckham,  is  a  further  in- 
stance of  the  unscientific  use  of  anecdotal  testimony. 

An  illustration  of  the  third  objection  mentioned  above, 
the  disadvantage  of  ignorance  of  the  animal's  individual  his- 
tory, is  furnished  by  Lloyd  Morgan.  In  describing  his  futile 
efforts  to  teach  a  fox  terrier  the  best  way  to  pull  a  crooked 
stick  through  a  fence,  he  says  that  the  dog  showed  no  sign 
"of  perceiving  that  by  pushing  the  stick  and  freeing 
the  crook  he  could  pull  the  stick  through.  Each  time  the 
crook  caught  he  pulled  with  all  his  strength,  seizing  the 
stick  now  at  the  end,  now  in  the  middle,  and  now  near 
the, crook.  At  length  he  seized  the  crook  itself  and  with 
a  wrench  broke  it  off.  A  man  who  was  passing  .  .  .  said, 
' Clever  dog  that,  sir;  he  knows  where  the  hitch  do  lie.' 
The  remark  was  the  characteristic  outcome  of  two  minutes' 
chance  observation"  (507,  pp.  142-143).  How  many 
anecdotes  of  animals  are  based  on  similar  accidents  ? 

It  will  be  seen  that  in  both  the  cases  just  criticised  the 
error  lies  in  the  interpretation  of  the  animal's  behavior. 
Indeed,  a  root  of  evil  in  the  method  of  anecdote  consists 
in  the  fact  that  observation  in  this  form  is  imperfectly 
divorced  from  interpretation.  The  maker  of  an  anecdote 


8  The  Animal  Mind 

is  seldom  content  with  merely  telling  one  what  the  animal 
did  and  leaving  future  investigation  and  the  comparative 
study  of  many  facts  to  decide  what  the  animal's  conscious 
experience  in  doing  it  was  like.  The  point  of  the  anecdote 
usually  consists  in  showing  that  a  human  interpretation 
of  the  animal's  behavior  is  possible.  Here  is  shown  the 
desire  to  tell  a  good  story,  which  we  mentioned  among 
the  pitfalls  of  the  anecdotal  method;  the  wish  to  report 
something  unusual,  not  to  get  a  just  conception  of  the 
normal  behavior  of  an  animal.  As  Thorndike  has  forcibly 
put  it :  "Dogs  get  lost  hundreds  of  times  and  no  one  ever 
notices  it  or  sends  an  account  of  it  to  a  scientific  maga- 
zine. But  let  one  find  his  way  from  Brooklyn  to  Yonkers 
and  the  fact  immediately  becomes  a  circulating  anecdote. 
Thousands  of  cats  on  thousands  of  occasions  sit  helplessly 
yowling,  and  no  one  takes  thought  of  it  or  writes  to  his  friend 
the  professor ;  but  let  one  cat  claw  at  the  knob  of  a  door 
supposedly  as  a  signal  to  be  let  out,  and  straightway  this 
cat  becomes  the  representative  of  the  cat-mind  in  all  the 
books"  (704,  p.  4). 

All  this  is  not  to  deny  that  much  of  the  testimony  to  be 
found  in  Romanes's  "Animal  Intelligence"  and  Darwin's 
"Descent  of  Man"  is  the  trustworthy  report  of  trained  ob- 
servers; but  it  is  difficult  to  separate  the  grain  from  the 
chaff,  and  one  feels  toward  many  of  the  anecdotes  the  atti- 
tude of  scepticism  produced,  for  example,  by  this  tale 
which  an  Australian  lady  reported  to  the  Linnaean  Society. 
The  burial  of  some  deceased  comrades  was  accomplished, 
she  says,  by  a  nest  of  "soldier  ants"  near  Sydney,  in  the 
following  fashion.  "All  fell  into  rank  walking  regularly 
and  slowly  two  by  two,  until  they  arrived  at  the  spot 
where  lay  the  dead  bodies.  .  .  .  Two  of  the  ants  advanced 
and  took  up  the  dead  body  of  one  of  their  comrades ;  then 


Difficulties  and  Methods  9 

two  others,  and  so  on  until  all  were  ready  to  march.  First 
walked  two  ants  bearing  a  body,  then  two  without  a  burden ; 
then  two  others  with  another  dead  ant,  and  so  on,  until 
the  line  was  extended  to  about  forty  pairs,  and  the  pro- 
cession now  moved  slowly  onward,  followed  by  an  irregular 
body  of  about  two  hundred  ants.  Occasionally  the  two 
laden  ants  stopped,  and  laying  down  the  dead  ant,  it  was 
taken  up  by  the  two  walking  unburdened  behind  them, 
and  thus,  by  occasionally  relieving  each  other,  they  arrived 
at  a  sandy  spot  near  the  sea."  A  separate  grave  was 
then  dug  for  each  dead  ant.  "Some  six  or  seven  of  the 
ants  had  attempted  to  run  off  without  performing  their 
share  of  the  task  of  digging ;  these  were  caught  and  brought 
back,  when  they  were  at  once  attacked  by  the  body  of 
ants  and  killed  upon  the  spot.  A  single  grave  was  quickly 
dug  and  they  were  all  dropped  into  it."  No  funeral  pro- 
cession for  them!  Of  this  story  Romanes  says,  "The 
observation  seems  to  have  been  one  about  which  there 
could  scarcely  have  been  a  mistake"  (641,  p.  91).  One 
is  inclined  to  think  it  just  possible  that  there  was. 

§  3 .   Methods  of  Obtaining  Facts :  The  Method  of  Experiment 

Diametrically  opposed  to  the  Method  of  Anecdote  and 
its  unscientific  character  is  the  Method  of  Experiment. 
An  experiment,  properly  conducted,  always  implies  that 
the  conditions  are  controlled,  or  at  least  known ;  whereas 
ignorance  of  the  conditions  is,  as  we  have  seen,  a  common 
feature  of  anecdote.  The  experimenter  is  impartial;  he 
has  no  desire  to  bring  about  any  particular  result.  The 
teller  of  an  anecdote  wishes  to  prove  animal  intelligence. 
The  experimenter  is  willing  to  report  the  facts  precisely  as 
he  observes  them,  and  is  in  no  haste  to  make  them  prove 


io  The  Animal  Mind 

anything.  The  conduct  of  an  experiment  upon  an  animal 
will,  of  course,  vary  according  to  the  problem  to  be  solved. 
If  the  object  is  to  test  some  innate  reaction  on  the  animal's 
part,  such  as  its  ordinary  responses  to  stimulation  or  its 
instincts,  one  need  merely  place  the  animal  under  favorable 
conditions  for  observation,  make  sure  that  it  is  not  fright- 
ened or  in  an  abnormal  state,  supply  the  appropriate  stimu- 
lus unmixed  with  others,  and  watch  the  result.  If  it  is 
desired  to  study  the  process  by  which  an  animal  learns  to 
adapt  itself  to  a  new  situation,  one  must,  of  course,  make 
sure  in  addition  that  the  situation  really  is  new  to  the  ani- 
mal, and  yet  that  it  makes  sufficient  appeal  to  some  instinc- 
tive tendency  to  supply  a  motive  for  the  learning  process. 

As  one  might  expect,  among  the  earliest  experiments 
upon  animals  were  those  made  by  physiologists  with  a  view 
to  determining  the  functions  of  sense  organs.  The  experi- 
mental movement  in  psychology  was  slow  in  extending 
itself  into  the  field  of  the  animal  mind. 

Romanes,  whose  adherence  to  the  anecdotal  method  we 
have  noted,  made  in  1881,  rather  as  a  physiologist  than  as 
a  psychologist,  a  number  of  exact  and  highly  valued  experi- 
ments on  ccelenterates  and  echinoderms,  which  were  sum- 
marized in  his  book  entitled  "  Jelly-fish,  Star-fish,  and  Sea- 
urchins,"  published  in  1885.  He  has  also  recorded  some 
rather  informal  experiments  on  the  keenness  of  smell  in 
dogs.  Sir  John  Lubbock,  in  1883,  reported  the  results  of 
some  experiments  on  the  color  sense  of  the  small  crustacean 
Daphnia,  and  his  book  on  "Ants,  Bees,  and  Wasps,"  con- 
taining an  account  of  experimental  tests  of  the  senses  and 
"intelligence"  of  these  insects,  appeared  in  the  same  year. 
A  German  entomologist,  Vitus  Graber,  experimented  very 
extensively  at  about  this  period  on  the  senses  of  sight  and 
smell  in  many  animals.  Preyer,  the  authority  on  child 


Difficulties  and  Methods  n 

psychology,  published  in  1886  an  experimental  study  of  the 
behavior  of  the  starfish.  Loeb's  work  on  the  reactions  of 
animals  to  stimulation  began  to  appear  in  1888.  Max 
Verworn,  the  physiologist,  published  in  1889  an  exhaustive 
experimental  study  of  the  behavior  of  single-celled  animals. 
With  the  exception  of  Preyer  and  Romanes,  all  these  men 
had  but  a  secondary  interest  in  comparative  psychology : 
Bethe,  indeed,  as  we  shall  see,  wholly  rejects  it.  Lloyd 
Morgan,  who  has  written  instructively  on  comparative 
psychology,  makes  but  a  limited  use  of  the  experimental 
method.  Wesley  Mills,  professor  of  physiology  in  McGill 
University,  has  studied  very  carefully  the  mental  develop- 
ment of  young  animals  such  as  cats  and  dogs,  but  is  inclined 
to  criticise  the  use  of  experiment  in  observing  animals. 
The  work  of  E.  L.  Thorndike,  whose  "Animal  Intelligence" 
appeared  in  1898,  represents,  perhaps,  the  first  definite 
effect  of  the  modern  experimental  movement  in  psychology 
upon  the  study  of  the  animal  mind.  Thorndike 's  aim  in 
this  research  was  to  place  his  animals  (chicks,  cats,  and 
dogs)  under  the  most  rigidly  controlled  experimental  condi- 
tions. The  cats  and  dogs,  reduced  by  fasting  to  a  state  of 
"utter  hunger,"  were  placed  in  boxes,  with  food  outside, 
and  the  process  whereby  they  learned  to  work  the  various 
mechanisms  which  let  them  out  was  carefully  observed. 
Since  the  appearance  of  Thorndike 's  work  the  performance 
of  experiments  upon  animals  has  played  much  part  in  the 
.work  of  psychological  laboratories,  particularly  those  of 
Harvard,  Clark,  and  Chicago  universities.  The  biologists 
and  physiologists  have  continued  their  researches  by  this 
method,  so  that  a  very  large  amount  of  experimental 
work  is  now  being  done  in  comparative  psychology. 

Despite   the   obvious   advantages   of  experiment  as   a 
method  for  the  study  of  animal  behavior,  it  is  not  without 


12  The  Animal  Mind 

its  dangers.  These  were  clearly  stated  by  Wesley  Mills 
in  a  criticism  of  Thorndike's  "Animal  Intelligence"  (492). 
They  may  be  summed  up  by  saying  that  there  is  a  risk  of 
placing  the  animal  experimented  upon  under  abnormal  con- 
ditions in  the  attempt  to  make  them  definite  and  control- 
lable.1 Did  not,  for  example,  the  extreme  hunger  to  which 
Thorndike's  cats  and  dogs  were  reduced,  while  it  simplified 
the  conditions  in  one  sense  by  making  the  strength  of  the 
motive  to  escape  as  nearly  as  possible  equal  for  all  the 
animals,  complicate  matters  in  another  sense  by  diminish- 
ing their  capacity  to  learn?  Were  the  animals  perhaps 
frightened  and  distracted  by  the  unusual  character  of  their 
surroundings  ?  Thorndike  thinks  not  (707)  ;  but  whether 
or  no  he  succeeded  in  averting  these  dangers,  it  is  clear 
that  they  are  real.  It  is  also  obvious  that  they  are  the 
more  threatening,  the  higher  the  animal  with  which  one 
has  to  deal.  Fright,  bewilderment,  loneliness,  are  condi- 
tions more  apt  to  be  met  with  among  the  higher  verte- 
brates than  lower  down  in  the  scale,  and  the  utmost  care 
should  be  taken  to  make  sure  that  animals  likely  to  be 
affected  by  them  are  thoroughly  trained  and  at  home  in 
their  surroundings  before  the  experimenter  records  results. 

§  4.   Methods  of  Obtaining  Facts:    The  Ideal  Method 

The  ideal  method  for  the  study  of  a  higher  animal 
involves  patient  observation  upon  a  specimen  known  from 
birth,  watched  in  its  ordinary  behavior  and  environment, 
and  occasionally  experimented  upon  with  proper  control 
of  the  conditions  and  without  frightening  it  or  otherwise 
rendering  it  abnormal.  The  observer  should  acquaint 
himself  with  the  individual  peculiarities  of  each  animal 

JCf.  also  Kline  (402),  and  Vaschide  and  Rousseau  (739). 


Difficulties  and  Methods  13 

studied,  for  there  is  no  doubt  that  striking  differences  in 
mental  capacity  occur  among  the  individuals  of  a  single 
species.  At  the  same  time  that  he  obtains  the  confidence 
of  each  individual  animal,  he  should  be  able  to  hold  in 
check  the  tendency  to  humanize  it  and  to  take  a  personal 
pleasure  in  its  achievements  if  it  be  unusually  endowed. 
This  is,  to  say  the  least,  not  easy.  Absolute  indifference 
to  the  animals  studied,  if  not  so  dangerous  as  doting  affec- 
tion, is  yet  to  be  avoided. 

§  5.   Methods  of  Interpreting  Facts 

We  may  now  turn  from  the  problem  of  discovering  the 
facts  about  animal  behavior  to  the  problem  of  interpreting 
them.  If  an  animal  behaves  in  a  certain  manner,  what 
may  we  conclude  the  consciousness  accompanying  its 
behavior  to  be  like?  As  we  have  seen,  the  interpretation 
is  often  confused  with  the  observation,  especially  in  the 
making  of  anecdotes ;  but  theoretically  the  two  problems 
are  distinct.  And  at  the  outset  of  our  discussion  of  the 
former,  we  are  obliged  to  acknowledge  that  all  psychic 
interpretation  of  animal  behavior  must  be  on  the  analogy  of 
human  experience.  We  do  not  know  the  meaning  of 
such  terms  as  perception,  pleasure,  fear,  anger,  visual 
sensation,  etc.,  except  as  these  processes  form  a  part  of 
the  contents  of  our  own  minds.  Whether  we  will  or  no, 
we  must  be  anthropomorphic  in  the  notions  we  form  of 
what  takes  place  in  the  mind  of  an  animal.  Accepting 
this  fundamental  proposition,  the  students  of  animals 
have  yet  differed  widely  in  the  conclusions  they  have 
drawn  from  it.  Some  have  gone  to  the  extreme  of  declar- 
ing that  comparative  psychology  is  therefore  impossible. 
Others  have  joyfully  hastened  to  make  animals  as  human 


14  The  Animal  Mind 

as  they  could.     Still  others  have  occupied  an  intermediate 
position. 

Descartes  and  Montaigne  are  the  two  writers  antedating 
the  modern  period  who  are  most  frequently  quoted  in  this 
connection.  The  latter  had  evidently  a  natural  sympathy 
with  animals.  In  that  most  delightful  twelfth  chapter  of 
the  second  book  of  Essays,  "An  Apology  of  Raymond 
Sebonde,"  he  gives  free  rein  to  the  inclination  to  humanize 
them.  I  quote  Florio's  translation:  "The  Swallowes 
which  at  the  approach  of  spring  time  we  see  to  pry,  to 
search  and  ferret  all  the  corners  of  our  houses ;  is  it  with- 
out judgment  they  seeke,  or  without  discretion  they  chuse 
from  out  a  thousand  places,  that  which  is  fittest  for  them, 
to  build  their  nests  and  lodging?  .  .  .  Would  they  (sup- 
pose you)  first  take  water  and  then  clay,  unlesse  they 
guessed  that  the  hardnesse  of  the  one  is  softned  by  the 
moistness  of  the  other?  .  .  .  Why  doth  the  spider  spin 
her  artificiall  web  thicke  in  one  place  and  thin  in  another  ? 
And  now  useth  one,  and  then  another  knot,  except  she 
had  an  imaginary  kind  of  deliberation,  forethought,  and 
conclusion?"  To  ascribe  such  behavior  to  the  working 
of  mere  instinct,  "with  a  kinde  of  unknowne,  naturall  and 
servile  inclination,"  is  unreasonable.  "The  Fox,  which 
the  inhabitants  of  Thrace  use"  to  test  the  ice  on  a  river 
before  crossing,  which  listens  to  the  roaring  of  the  water 
underneath  and  so  judges  whether  the  ice  is  safe  or  not; 
"might  not  we  lawfully  judge  that  the  same  discourse  pos- 
sesseth  her  head  as  in  like  case  it  would  ours?  And  that 
it  is  a  kinde  of  debating  reason  and  consequence,  drawne 
from  natural  sense  ?  '  Whatsoever  maketh  a  noyse  moveth, 
whatsoever  moveth,  is  not  frozen,  whatsoever  is  not  frozen, 
is  liquid ;  whatsoever  is  liquid,  yeelds  under  any  weight  ? ' ' 
(498). 


Difficulties  and  Methods  15 

Descartes,  on  the  other  hand,  writing  some  sixty  years 
later,  takes,  as  is  well  known,  the  opposite  ground.  He 
says  in  a  letter  to  the  Marquis  of  Newcastle,  "As  for  the 
understanding  or  thought  attributed  by  Montaigne  and 
others  to  brutes,  I  cannot  hold  their  opinion."  While 
animals  surpass  us  in  certain  actions,  it  is,  he  holds,  only 
in  those  "  which  are  not  directed  by  thought.  .  .  .  They 
act  by  force  of  nature  and  by  springs,  like  a  clock,  which 
tells  better  what  the  hour  is  than  our  judgment  can  inform 
us.  And  doubtless  when  swallows  come  in  the  spring, 
they  act  in  that  like  clocks.  All  that  honey  bees  do  is 
of  the  same  nature"  (183,  pp.  281-283).  The  statement 
of  Descartes,  contained  in  the  letter  to  Mersenne  of  July 
30,  1640,  that  animals  are  automata,  is  often  misunder- 
stood. Descartes  does  not  assert  that  animals  are  uncon- 
scious in  the  sense  which  that  term  would  carry -to-day, 
but  only  that  they  are  without  thought.  Sensations,  feel- 
ings, passions,  he  is  willing  to  ascribe  to  them,  in  so  far  as 
these  do  not  involve  thought.  "It  must  however  be 
observed  that  I  speak  of  thought,  not  of  life,  nor  of  sensa- 
tion," he  says  in  the  letter  to  Henry  More,  1649;  "I  do 
not  refuse  to  them  feeling  ...  in  so  far  as  it  depends 
only  on  the  bodily  organs"  (183,  p.  287).  In  this  he  does 
not  go  so  far  as  some  modern  writers,  who  decline  to  assert 
the  presence  of  any  psychic  process  in  the  lower  forms  of 
animal  life. 

Turning  to  recent  times,  we  find  arguments  very  like 
those  of  Montaigne  used  by  the  earlier  evolutionary  writers. 
Darwin,  for  instance,  says  in  "The  Descent  of  Man,"  "As 
dogs,  cats,  horses,  and  probably  all  the  higher  animals,  even 
birds,  have  vivid  dreams,  and  this  is  shown  by  their  move- 
ments and  the  sounds  uttered,  we  must  admit  that  they  pos- 
sess some  power  of  imagination"  (169,  p.  74).  "Even 


1 6  The  Animal  Mind 

brute  beasts/'  says  Montaigne,  ".  .  .  are  seen  to  be  sub- 
ject to  the  power  of  imagination ;  witnesse  some  Dogs  .  .  . 
whom  we  ordinarily  see  to  startle  and  barke  in  their  sleep" 
(501,  Bk.  I,  ch.  20).  "Only  a  few  persons,"  Darwin  con- 
tinues, "now  dispute  that  animals  possess  some  power  of 
reasoning.  Animals  may  constantly  be  seen  to  pause, 
deliberate,  and  resolve."  And  he  states  that  his  object 
in  the  third  chapter  of  the  work  quoted  is  "to  show  that 
there  is  no  fundamental  difference  between  man  and  the 
higher  mammals  in  their  mental  faculties"  (169,  p.  66). 
Romanes  is  evidently  guided  by  the  same  desire  to  humanize 
animals. 

Now  these  writers  were  not  led  to  take  such  an  attitude 
merely  out  of  general  sympathy  with  the  brute  creation, 
like  Montaigne;  they  had  an  ulterior  motive;  namely, 
to  meet  the  objection  raised  in  their  time  against  the 
doctrine  of  evolution,  based  on  the  supposed  fact  of  a 
great  mental  and  moral  gulf  between  man  and  the  lower 
animals.  They  wished  to  show,  as  Darwin  clearly  states, 
that  this  gulf  is  not  absolute  but  may  conceivably  have 
been  bridged  by  intermediate  stages  of  mental  and  moral 
development.  While  this  argument  against  evolution 
was  being  pressed,  the  evolutionary  writers  were  very 
unsafe  guides  in  the  field  of  animal  psychology,  for  they 
distinctly  "held  a  brief  for  animal  intelligence,"  to  use 
Thorndike's  phrase.  In  more  recent  times  interest  in  both 
the  positive  and  the  negative  sides  of  the  objection  drawn 
from  man's  superiority  has  died  out,  and  such  special 
pleading  has  become  unnecessary. 

On  the  other  hand,  the  fact  that  the  greater  part  of  the 
experiments  on  animals  were  until  the  last  ten  or  fifteen 
years  performed  by  physiologists  has  given  rise  to  an 
opposite  tendency  in  interpreting  the  animal  mind:  the 


Difficulties  and  Methods  17 

tendency  to  make  purely  biological  concepts  suffice  as 
far  as  possible  for  the  explanation  of  animal  behavior  and 
to  assume  the  presence  even  of  consciousness  in  animals 
only  when  it  is  absolutely  necessary  to  do  so.  Logb  in 
1890  suggested  the  theory  which  he  has  since  elaborated, 
that  the  responses  of  animals  to  stimulation,  instead  of 
being  signs  of  "sensation,"  are  in  every  way  analogous  to 
the  reactions  of  plants  to  such  forces  as  light  and  gravity ; 
hence  unconscious  "tropisms"  (421).  Bethe  in  1898 
attempted  to  explain  all  the  complicated  behavior  of  ants 
and  bees,  which  the  humanizing  writers  had  compared 
with  our  own  civilization,  as  a  result  of  reflex  responses, 
chiefly  to  chemical  stimulation,  unaccompanied  by  any 
consciousness  whatever  (51).  This  revival,  in  an  altered 
form,  of  the  Cartesian  doctrine  has  met  with  energetic 
opposition,  especially  from  writers  having  philosophical 
interests.  When  the  first  edition  of  the  present  work 
appeared,  the  parties  in  the  controversy  could  be  divided 
into  three  groups :  those  who  believed  that  consciousness 
should  be  ascribed  to  all  animals;  those  who  believed 
that  it  should  be  ascribed  only  to  those  animals  whose 
behavior  presents  certain  peculiarities  regarded  as  evidence 
of  mind ;  and  those  who  held  that  we  have  no  trustworthy 
evidence  of  mind  in  any  animal,  and  should  therefore 
abandon  comparative  psychology  and  use  only  physio- 
logical terms.  Of  recent  years,  the  tendency  has  been 
towards  the  survival  only  of  the  two  extreme  parties :  it 
has  been  more  and  more  recognized  that  there  exists  no 
evidence  of  mind  which  is  not  either  equally  bad  or  equally 
good  in  the  case  of  all  animals. 

Among  the  authorities  who  would  ascribe  mind  to  all 
animals  belong  Claparede  of  Geneva,  the  Swiss  natu- 
ralist Forel,  and  the  Jesuit  Wasmann.  They  maintain 


1 8  The  Animal  Mind 

this  position  from  widely  different  philosophical  points 
of  view.  The  first-named  is  what  is  called  a  parallel- 
ist;  that  is,  he  believes  that  mental  processes  and  bodily 
processes  are  not  causally  related,  but  form  two  parallel  and 
non-interfering  series  of  events.  In  the  study  of  animals, 
both  the  physical  and  the  psychical  series  should,  he  thinks, 
be  investigated.  Biology  should  use  two  parallel  methods, 
the  one  ascending,  attempting  to  explain  animal  behavior  by 
physical  and  chemical  laws;  the  other  descending,  giving 
an  account  of  the  mental  processes  of  animals.  Ultimately, 
it  may  be  hoped,  according  to  Claparede,  that  both  methods 
will  be  applied  throughout  the  whole  range  of  animal  life. 
At  present  the  ascending  method  is  most  successful  with  the 
lowest  forms,  the  descending  method  with  the  highest  forms. 
We  cannot  afford  to  abandon  the  psychological  study  of 
animals,  for  our  knowledge  of  the  nervous  processes  under- 
lying the  higher  mental  activities  is  very  slight ;  physiology 
here  fails  us,  and  psychology  must  be  left  in  command  of 
the  field.  The  danger  besetting  the  attempt  at  a  purely 
physical  explanation  of  animal  behavior  is  that  the  facts 
shall  be  unduly  simplified  to  fit  the  theory.  Thus  Bethe's 
effort  at  explaining  the  way  in  which  bees  find  their  way 
back  to  the  hive  as  a  reflex  response,  or  tropism,  produced 
by  "an  unknown  force,"  is  highly  questionable;  the  facts 
seem  to  point  toward  the  exercise  of  some  sort  of  memory 
by  the  bees.  It  is  always  possible,  further,  that  the  tropism 
is  accompanied  by  consciousness.  A  physiologist  from 
Saturn  might  reduce  all  human  activities  to  tropisms,  says 
Claparede  in  a  striking  passage.  "The  youth  who  feels 
himself  drawn  to  medical  studies,  or  he  who  is  attracted 
to  botany,  can  no  more  account  for  his  profoundest  aspira- 
tions than  the  beetle  which  runs  to  the  odor  of  a  dead 
animal  or  the  butterfly  invited  by  the  flowers ;  and  if  the 


Difficulties  and  Methods  19 

first  shows  a  certain  feeling  corresponding  to  these  secret 
states  of  the  organism  (a  feeling  of  'predilection'  for  such 
a  career,  etc.),  how  can  we  dare  to  deny  to  the  second 
analogous  states  of  consciousness?"  (122).  If  it  is  argued 
that  we  have  no  direct,  but  only  an  inferential,  knowl- 
edge of  the  processes  in  an  animal's  mind,  the  argument 
is  equally  valid  against  human  psychology,  for  the  psychol- 
ogist has  only  an  inferential  knowledge  of  his  neighbor's 
mind  (124). 

Wasmann  defends  the  animal  mind  from  a  different 
position.  For  one  thing,  he  believes  that  mental  processes 
may  act  causally  upon  bodily  states.  He  accepts,  in  other 
words,  what  is  called  n^mc_tionism,  as  opposed  to  parallel- 
ism. Further,  although  he  strongly  opposes  the  doctrine 
that  the  reactions  of  animals  are  unconscious  tropisms  and 
constantly  emphasizes  their  variability  and  modifiability 
through  experience,  he  nevertheless  believes  that  a  gulf 
separates  the  human  from  the  animal  mind.  The  term 
"  intelligence "  which  most  writers  use  to  designate  merely 
the  power  of  learning  by  individual  experience,  Wasmann 
would  reserve  for  the  power  of  deducing  and  understand- 
ing relations,  and  would  assign  only  to  human  beings 
(761,  762).  Although  animals  have  their  instincts  modified 
by  sense  experience,  man  "  stands  through  his  reason  and 
freedom  immeasurably  high  above  the  irrational  animal 
that  follows,  and  must  follow,  its  sensuous  impulse  without 
deliberation"  (763). 

For  el,  in  the  third  place,  is  what  is  called  a  monist  in 
metaphysics.  That  is,  he  does  not  believe  either  that  mind 
and  body  are  parallel,  or  that  they  interact  causally,  but 
that  they  are  two  aspects  of  the  same  reality.  "  Every  psy- 
chic phenomenon  is  the  same  real  thing  as  the  molecular 
or  neurocymic  activity  of  the  brain-cortex  coinciding  with 


2o  The  Animal  Mind 

ft"  (233>  P-  ?)•  The  psychic  and  the  physical,  on  this 
theory,  should  be  coextensive;  not  merely  should  con- 
sciousness in  some  form  belong  to  all  living  things,  but 
every  atom  of  matter  should  have  its  psychic  aspect. 
On  such  a  basis,  Forel  takes  highly  optimistic  views  of 
the  animal  mind.  In  insects,  of  which  he  has  made  a 
special  study,  it  is,  he  thinks,  "  possible  to  demonstrate 
the  existence  of  memory,  associations  of  sensory  images, 
perceptions,  attention,  habits,  simple  powers  of  inference 
from  analogy,  the  utilization  of  individual  experience,  and 
hence  distinct,  though  feeble,  plastic  individual  delibera- 
tions or  adaptations"  (233,  p.  36). 

A  peculiar  position  on  the  problem  of  mind  in  animals 
is  occupied  by  the  "vitalists, "  of  whom  Driesch  (191) 
is  the  foremost  representative.  They  regard  the  reactions 
of  organisms  as  requiring  the  operation  of  psychic  forces 
or  "  entelechies " ;  they  hold  that  as  physical  phenomena 
such  reactions  cannot  be  explained  save  through  the  work- 
ing of  these  psychic  forces.  A  living  being  is  forever  dis- 
tinguished from  a  lifeless  creature  by  the  presence  of  such 
entelechies.  Thus  the  vitalist  is  an  interactionist  and  a 
dualist :  the  worlds  of  the  lifeless  and  the  living  are  to  him 
forever  distinct. 

The  opposite  camp  is  represented  by  Bethe,  Beer,  von 
Uexkiill,  Loeb,  and  other  physiologists,  as  well  as  by 
Watson. 

The  eminent  neurologist  Bethe,  in  his  study  of  the  be- 
havior of  ants  and  bees,  refuses  to  allow  these  animals  any 
"psychic  qualities"  whatever,  and  suggests  the  term 
" chemo-reception "  instead  of  "smell,"  to  designate  the 
influence  which  directs  most  of  their  reactions,  —  "smell" 
implying  a  psychic  quality  (51).  In  a  footnote  to  a  later 
article  he  says :  "Psychic  qualities  cannot  be  demonstrated. 


Difficulties  and  Methods  21 

Even  what  we  call  sensation  is  known  to  each  man  only 
in  himself,  since  it  is  something  subjective.  We  possess 
the  capacity  of  modifying  our  behavior  [i.e.  of  learning], 
and  every  one  knows  from  his  own  experience  that  psychic 
qualities  play  a  part  connected  with  this  modifying  process. 
Every  statement  that  another  being  possesses  psychic 
qualities  is  a  conclusion  from  analogy,  not  a  certainty ;  it 
is  a  matter  of  faith.  If  one  wishes  to  draw  this  analogical 
inference,  it  should  be  made  where  the  capacity  for  modi- 
fication can  be  shown.  When  this  is  lacking,  there  is  not 
the  slightest  scientific  justification  for  assuming  psychic 
qualities.  They  may  exist,  but  there  is  no  probability 
of  it,  and  hence  science  should  deny  them.  Hence  if  one 
ventures  to  speak  of  a  Psych  in  animals  at  all,  one  should 
give  the  preference  to  those  which  can  modify  their  be- 
havior" (51).  But  that  Bethe  himself  prefers  not  to  make 
the  venture  is  evident  from  statements  in  the  text  of  the 
same  article.  The  psychic  or  subjective,  he  says,  is  un- 
knowable, and  the  only  thing  we  may  hope  to  know  any- 
thing about  is  the  chemical  and  physiological  processes 
involved.  "These  chemo-physical  processes  and  their 
consequences,  that  is,  the  objective  aspect  of  psychic 
phenomena,  and  these  alone,  should  be  the  object  of 
scientific  investigation"  (51). 

Together  with  Beer  and  von  Uexklill,  Bethe  shortly 
afterward  published  "  Proposals  for  an  Objectifying  Nomen- 
clature in  the  Physiology  of  the  Nervous  System."  The 
main  purpose  of  this  paper  was  to  suggest  that  all  terms 
having  a  psychological  implication,  such  as  sight,  smell, 
sense-organ,  memory,  learning,  and  the  like,  be  carefully 
excluded  from  discussions  of  animal  reactions  to  stimula- 
tion and  animal  behavior  generally.  In  their  stead  the 
authors  propose  such  expressions  as  the  following:  for 


22  The  Animal  Mind 

responses  to  stimulation  where  no  nervous  system  exists, 
the  term  antitypes;  for  those  involving  a  nervous  system, 
antikineses ;  the  latter  are  divided  into  reflexes,  where 
the,  response  is  uniform,  and  antiklises,  where  the  response 
is  modifiable.  A  sense-organ  becomes  a  reception-organ, 
sensory  nerves  are  receptory-nerves,  and  we  have  phono- 
reception,  stibo-reception,  photo-reception,  instead  of  hear- 
ing, smell,  and  sight.  The  after-effect  of  a  stimulus  upon 
later  ones  is  the  resonance  of  the  stimulus  (39). 

Loeb  (434)  agrees  with  Be  the  that  physico-chemical  pro- 
cesses and  not  states  of  consciousness  are  the  proper  objects 
of  investigation  for  the  " psychologist."  These  men  evi- 
dently regard  the  universe  as  essentially  uniform  through- 
out —  there  exists  for  them  no  gulf  between  living  and  life- 
less things;  the  behavior  of  living  beings  will  be  reduced 
to  a  series  of  chemical  reactions  as  soon  as  science  has  pro- 
gressed sufficiently  far.  They  are  "mechanists."  It  is, 
however,  perfectly  possible  to  be  a  mechanist  so  far  as 
the  explanation  of  animal  behavior  is  concerned,  and  still 
admit  that  animals  have  consciousness  and  that  their 
behavior  is  accompanied  by  inner,  mental  states  which 
it  is  the  business  of  the  psychologist  to  investigate.  One 
does  not  have  to  be  a  vitalist  to  believe  that  animals  have 
minds :  one  may  hold  that  every  action  of  an  animal 
will  some  day  be  explained  as  the  result  of  physico-chemical 
processes,  and  yet  maintain  that  the  actions  of  animals 
are  conscious.  The  consciousness  would  be  an  accompani- 
ment, an  inner  aspect,  of  the  physico-chemical  processes. 

The  views  of  Loeb  and  Bethe  have  gained  much  ground 
lately  among  certain  American  psychologists,  notably 
Watson  (771).  The  position  of  these  " behaviorists " 
seems  not  to  have  been  fully  thought  out  in  its  philosophical 
aspects,  but  is  somewhat  as  follows.  The  difficulties  of 


Difficulties  and  Methods  23 

interpreting  an  animal's  mind  from  its  behavior  are  so 
great  that  such  inferences  have  no  scientific  value.  We 
may  therefore  proceed  as  if  animals  had  no  minds;  or 
rather,  as  if  mind  were  a  kind  of  behavior,  observable 
by  outside  means.  Since  it  is  obvious  that  the  difficulty 
of  interpreting  an  animal's  mind  from  its  behavior  is  only 
greater  in  degree  than,  not  unlike  in  kind,  the  difficulty 
of  interpreting  other  human  minds  from  behavior,  human 
psychology  also  should  confine  itself  to  the  observation 
merely  of  the  actions  of  other  persons,  and  permit  no  infer- 
ences as  to  the  inner  aspect  of  such  actions.  In  fact, 
there  is  no  inner  aspect  to  such  actions  —  thoughts  and 
feelings,  human  as  well  as  animal,  are  only  behavior,  and 
if  we  have  at  present  no  instruments  for  inspecting  and 
measuring  the  movements  which  are  thoughts  and  feelings, 
such  instruments  will  in  time  be  discovered. 

In  opposition  to  these  views,  we  shall  in  this  book  main- 
tain the  following  position.  There  exists  an  inner  aspect 
to  behavior,  the  realm  of  sensations,  feelings,  and  thoughts, 
which  is  not  itself  identical  with  behavior  or  with  any  form 
of  movement.  Thoughts  probably  always  have  as  their 
accompaniment  bodily  movements,  but  the  thought  is  not 
identical  with  the  movement.  If  a  physiologist  perfected 
an  instrument  by  which  he  could  observe  the  nervous 
process  in  my  cortex  that  occurs  when  I  am  conscious 
of  the  sensation  red,  he  would  see  nothing  red  about  it; 
if  he  could  watch  the  bodily  movements  that  result  from 
this  stimulation,  say,  for  instance,  the  slight  contraction  of 
the  articulatory  muscles  that  occurs  when  I  say  "red"  to 
myself,  he  would  not  see  them  as  red.  The  red  is  in  my 
consciousness,  and  no  devices  for  observing  and  register- 
ing my  movements  will  ever  observe  the  red,  though  they 
may  easily  lead  to  the  inference  that  it  exists  in  my  con- 


24  The  Animal  Mind 

sciousness.  And  precisely  the  same  is  true  of  all  my  sensa- 
tions, thoughts,  and  feelings. 

Since  an  inner  world  of  experience  exists,  we  may  legiti- 
mately try  to  investigate  it.  For  this  purpose  we  possess 
a  method,  which  is  called  introspection.  We  can,  that  is, 
attentively  and,  if  we  have  had  practice,  dispassionately 
and  scientifically,  observe  what  goes  on  in  our  own  con- 
sciousness when  we  receive  certain  stimuli  and  make  cer- 
tain movements.  Further,  we  can,  by  the  use  of  the  same 
kind  of  inference  from  one  case  to  another  similar  case, 
upon  which  all  scientific  generalization  is  based,  infer  that 
when  a  being  whose  structure  resembles  ours  receives 
the  same  stimulus  that  affects  us  and  moves  in  the  same 
way  as  a  result,  he  has  an  inner  experience  which  resembles 
our  own.  Finally,  we  may  extend  this  inference  to  the 
lower  animals,  with  proper  safeguards,  just  as  far  as  they 
present  resemblances  in  structure  and  behavior  to  ourselves. 
Our  object  in  this  book  will  always  be  the  interpretation 
of  the  inner  aspect  of  the  behavior  of  animals;  we  shall 
be  interested  in  what  animals  do  only  as  it  throws  light 
upon  what  they  feel.  To  the  true  psychologist,  no  chal- 
lenge is  so  enticing  as  that  presented  by  the  problem  of 
how  it  feels  to  be  another  person  or  another  animal ;  and 
although  we  must  sometimes  give  up  the  problem  in  despair, 
yet  we  have  also  our  successes.  We  have  wonderfully 
advanced,  within  the  last  twenty-five  years,  in  knowledge 
as  to  how  the  world  looks  from  the  point  of  view  of  our 
brother  animals. 

We  may  now  note  briefly  some  of  the  special  precau- 
tions that  must  be  observed  in  interpreting  the  conscious 
aspect  of  animal  behavior.  First,  there  is  no  doubt  that 
great  caution  should  be  used  in  regarding  the  quality  of 
a  human  conscious  process  as  identical  with  the  quality 


Difficulties  and  Methods  25 

of  the  corresponding  process  in  the  animal  mind.  For 
example,  we  might  say  with  a  fair  degree  of  assurance  that 
an  animal  consciously  discriminates  between  light  and 
darkness;  that  is,  receives  conscious  impressions  of  dif- 
ferent quality  from  the  two,  yet  the  mental  impression 
produced  by  white  light  upon  the  animal  may  be  very 
different  from  the  sensation  of  white  as  we  know  it,  and 
the  impression  produced  by  the  absence  of  light  very  dif- 
ferent from  our  sensation  of  black.  Black  and  white  may, 
for  all  we  know,  depend  for  their  quality  upon  some  sub- 
stance existing  only  in  the  human  retina. 

A  second  precaution  concerns  the  simplicity  or  complexity 
of  the  interpretation  put  upon  animal  behavior.  Lloyd 
Morgan,  in  his  "  Introduction  to  Comparative  Psychology," 
formulated  a  conservative  principle  of  interpretation  which 
has  often  been  quoted  as  "  Lloyd  Morgan's  Canon."  The 
principle  is  as  follows :  "In  no  case  may  we  interpret  an 
action  as  the  outcome  of  the  exercise  of  a  higher  psychical 
faculty,  if  it  can  be  interpreted  as  the  outcome  of  the  exer- 
cise of  one  which  stands  lower  in  the  psychological  scale" 
(505,  p.  53).  In  other  words,  when  in  doubt  take  the 
simpler  interpretation.  For  example,  a  dog  detected  in 
a  theft  cowers  and  whines.  One  possible  mental  accom- 
paniment of  this  behavior  is  remorse ;  the  dog  is  conscious 
that  he  has  fallen  below  a  moral  standard,  and  grieved' 
or  offended  his  master.  A  second  is  the  anticipation  of 
punishment;  the  dog  has  a  mental  representation  of  the 
consequences  of  his  action  upon  former  occasions,  and 
imagining  himself  likely  to  experience  them  anew,  is  terrified 
at  the  prospect.  A  third  possibility  is  that  the  dog's  pre- 
vious experience  of  punishment,  instead  of  being  revived  in 
the  form  of  definite  images,  makes  itself  effective  merely  in 
his  feelings  and  behavior ;  he  is  uncomfortable  and  fright- 


26  The  Animal  Mind 

ened,  he  knows  not  definitely  why.  It  is  evident  that 
these  three  possibilities  represent  three  different  grades 
of  complexity  of  mental  process,  the  first  being  by  far  the 
highest.  Lloyd  Morgan's  canon  enjoins  upon  us  in  such 
a  case  to  prefer  the  third  alternative,  provided  that  it  will 
really  account  for  the  dog's  behavior. 

Now  why  should  the  simplest  interpretation  be  pre- 
ferred? We  must  not  forget  that  the  more  complex  ones 
remain  in  the  field  of  possibility.  Dogmatic  assertions 
have  no  place  in  comparative  psychology.  We  cannot 
say  that  the  simplicity  of  an  hypothesis  is  sufficient  war- 
rant of  its  truth,  for  nature  does  not  always  proceed  by 
the  paths  which  seem  to  us  least  complicated.  The  fact 
is  that  Lloyd  Morgan's  principle  serves  to  counterbalance 
our  most  important  source  of  error  in  interpreting  animal 
behavior.  It  is  like  tipping  a  boat  in  one  direction  to 
compensate  for  the  fact  that  some  one  is  pulling  the  opposite 
gunwale.  We  must  interpret  the  animal  mind  humanly 
if  we  are  to  interpret  it  at  all.  Yet  we  know  that  it  differs 
from  the  human  mind,  and  that  the  difference  is  partly 
a  matter  of  complexity.  Let  us  therefore  take  the  least 
complex  interpretation  that  the  facts  of  animal  behavior 
will  admit,  always  remembering  that  we  may  be  wrong 
in  so  doing,  but  resting  assured  that  we  are,  upon  the  whole, 
on  the  safer  side.  The  social  consciousness  of  man  is 
very  strong,  and  his  tendency  to  think  of  other  creatures, 
even  of  inanimate  nature,  as  sharing  his  own  thoughts  and 
feelings,  has  shown  itself  in  his  past  to  be  almost  irresistible. 
Lloyd  Morgan's  canon  offers  the  best  safeguard  against  this 
natural  inclination,  short  of  abandoning  all  attempt  to 
study  the  mental  life  of  the  lower  animals. 


CHAPTER  II 

THE  EVIDENCE  OF  MIND 

§  6.  Inferring  Mind  from  Behavior 

IN  this  chapter  we  shall  try  to  show  that  there  exists  no 
evidence  for  denying  mind  to  any  animals,  if  we  do  not 
deny  it  to  all ;  in  other  words,  that  there  is  no  such  thing 
as  an  objective  proof  of  the  presence  of  mind,  whose 
absence  may  be  regarded  as  proof  of  the  absence  of  mind. 

To  begin  with,  can  it  be  said  that  when  an  animal  makes 
a  movement  in  response  to  a  certain  stimulus,  there  is  an 
accompanying  consciousness  of  the  stimulus,  and  that  when 
it  fails  to  move,  there  is  no  consciousness?  Is  response  to-** 
stimulation  evidence  of  consciousness  ?  In  the  case  of  man, 
we  know  that  absence  of  visible  response  does  not  prove 
that  the  stimulus  has  not  been  sensed ;  while  it  is  probable 
that  some  effect  upon  motor  channels  always  occurs  when 
consciousness  accompanies  stimulation,  the  effect  may  not^ 
be  apparent  to  an  outside  observer.  On  the  other  hand,  if 
movement  in  response  to  the  impact  of  a  physical  force  is 
evidence  of  consciousness,  then  the  ball  which  falls  under 
the  influence  of  gravity  and  rebounds  on  striking  the  floor 
is  conscious.  Nor  is  the  case  improved  if  we  point  out  that 
the  movements  which  animals  make  in  response  to  stimula- 
tion are  not  the  equivalent  in  energy  of  the  stimulus  applied; 
but  involve  the  setting  free  of  energy  stored  in  the  animal 
as  well.  True,  when  a  microscopic  animal  meets  an  obstacle 
in  its  swimming,  and  darts  backward,  the  movement  is 

27 


28  The: Animal  Mind 

not  a  mere  rebound;  it  implies  energy  contributed  by 
the  animal's  own  body.  But  just  so  an  explosion  of  gun- 
powder is  not  the  equivalent  in  energy  of  the  heat  of  the 
match,  the  stimulus.  Similarly  it  is  possible  to  think  of 
the  response  made  by  animals  to  external  stimuli  as  in- 
volving nothing  more  than  certain  physical  and  chemical 
processes  identical  with  those  existing  in  inanimate  nature. 

If  we  find  that  the  movements  made  by  an  animal  as  a 
result  of  external  stimulation  regularly  involve  withdrawal 
from  certain  stimuli  and  acceptance  of  others,  it  is  natural 
to  use  the  term  "choice"  in  describing  such  behavior.  But 
if  consciousness  is  supposed  to  accompany  the  exercise  of 
choice  in  this  sense,  then  consciousness  must  be  assumed 
to  accompany  the  behavior  of  atoms  in  chemical  combina- 
tions. When  hydrochloric  acid  is  added  to  a  solution  of 
silver  nitrate,  the  atoms  of  chlorine  and  those  of  silver  find 
each  other  by  an  unerring  " instinct"  and  combine  into  the 
white  precipitate  of  silver  chloride,  while  the  hydrogen 
and  the  nitric  acid  similarly  "choose"  each  other.  Nor 
can  the  fact  that  behavior  in  animals  is  adapted  to  an  end  be 
used  as  evidence  of  mind ;  for  "purposive"  reactions,  which 
contribute  to  the  welfare  of  an  organism,  are  themselves 
selective.  The  search  for  food,  the  care  for  the  young, 
and  the  complex  activities  which  further  welfare,  are  made 
up  of  reactions  involving  "choice"  between  stimuli;  and 
if  the  simple  "choice"  reaction  is  on  a  par  with  the  behavior 
of  chemical  atoms,  so  far  as  proof  of  consciousness  goes, 
then  adaptation  to  an  end,  apparent  purposiveness,  is  in  a 
similar  position. 

Thus  the  mere  fact  that  an  animal  reacts  to  stimulation, 
even  selectively  and  for  its  own  best  interests,  offers  no  evi- 
dence for  the  existence  of  mind  that  does  not  apply  equally 
well  to  particles  of  inanimate  matter.  Moreover,  there  is 


The  Evidence  of  Mind  29 

some  ground  for  holding  that  the  reactions  of  the  lowest 
animals  are  unconscious.  This  ground  consists  in  the  ap- 
parent lack  of  variability  which  characterizes  such  reactions. 
In  our  own  case,  we  know  that  certain  bodily  movements, 
those  of  digestion  and  circulation,  for  example,  are  normally 
carried  on  without  accompanying  consciousness,  and  that  in 
other  cases  where  there  is  consciousness  of  the  stimulus,  as 
in  the  reflex  knee-jerk,  it  occurs  after  the  movement  is 
initiated,  so  that  the  nervous  process  underlying  the  sensa- 
tion would  seem  to  be  immaterial  to  the  performance  of 
the  movement.  These  unconscious  reactions  in  human 
beings  are  characterized  by  their  relative  uniformity,  by 
the  absence  of  variation  in  their  performance.  Moreover, 
when  an  action  originally  accompanied  by  consciousness  is 
often  repeated,  it  tends,  by  what  is  apparently  one  and  the 
same  process,  to  become  unconscious  and  to  become  uni- 
form. There  is  consequently  reason  for  believing  that  when 
the  behavior  of  lower  animals  displays  perfect  uniformity, 
consciousness  is  not  present.  On  the  other  hand,  an 
important  reservation  must  be  made  in  the  use  of  this 
negative  test.  It  is  by  no  means  easy  to  be  sure  that  an 
animal's  reactions  are  uniform.  The  more  carefully  the 
complexer  ones  are  studied,  the  more  are  variability  and 
difference  brought  to  light  where  superficial  observation 
had  revealed  a  mechanical  and  automatic  regularity.  It  is 
quite  possible  that  even  in  the  simple,  apparently  fixed  re- 
sponse of  microscopic  animals  to  stimulation,  better  facili- 
ties for  observation  might  show  variations  that  do  not  now 
appear. 

This  matter  of  uniformity  versus  variability  suggests  a 
further  step  in  our  search  for  a  satisfactory  test  of  the  pres- 
ence of  mind.  Is  mere  variability  in  behavior,  mere 
irregularity  in  response,  to  be  taken  as  such  a  test  ?  Not  if 


30  The  Animal  Mind 

we  argue  from  our  own  experience.  While  that  portion 
of  our  own  behavior  which  involves  consciousness  shows 
more  irregularity  than  the  portion  which  does  not,  yet  the 
causes  of  the  irregularity  are  often  clearly  to  be  found  in 
physiological  conditions  with  which  consciousness  has  noth- 
ing to  do.  There  are  days  when  we  can  think  clearly  and 
recall  easily,  and  days  when  obscurities  refuse  to  vanish 
and  the  right  word  refuses  to  come;  days  when  we  are 
irritable  and  days  when  we  are  sluggish.  Yet  since  we  can 
find  nothing  in  our  mental  processes  to  account  for  this 
variability,  it  would  be  absurd  to  take  analogous  fluctua- 
tions in  animal  behavior  as  evidence  of  mind.  So  com- 
plicated a  machine  as  an  animal  organism,  even  if  it  be 
nothing  more  than  a  machine,  must  show  irregularities 
in  its  working. 

Behavior,  then,  must  be  variable,  but  not  merely  variable, 
to  give  evidence  of  mind.  The  criterion  most  frequently 
applied  to  determine  the  presence  or  absence  of  the  psychic 
is  a  variation  in  behavior  that  shows  definitely  the  result  of 
previous  individual  experience.  "Does  the  organism, "says 
Romanes,  "  learn  to  make  new  adjustments,  or  to  modify  old 
ones,  in  accordance  with  the  results  of  its  own  individual  ex- 
perience?" (641,  p.  4).  Loeb  declared  that  "the  funda- 
mental process  which  occurs  in  all  psychic  phenomena  as 
the  elemental  component"  is  "the  activity  of  the  as- 
sociative memory,  or  of  association,"  and  defines  asso- 
ciative memory  as  "that  mechanism  by  which  a  stimulus 
brings  about  not  only  the  effects  which  its  nature  and  the 
specific  structure  of  the  irritable  organ  call  for,  but  by 
which  it  brings  about  also  the  effects  of  other  stimuli  which 
formerly  acted  upon  the  organism  almost  or  quite  simulta- 
neously with  the  stimulus  in  question."  "If  an  animal 
can  be  trained,"  he  continued,  "if  it  can  learn,  it  possesses 


The  Evidence  of  Mind  31 

associative  memory,"  and  therefore  mind  (429,  p.  12). 
The  psychologist  finds  the  term  "associative  memory" 
hardly  satisfactory,  and  objects  to  the  confusion  between 
mental  and  physical  concepts  which  renders  it  possible  to 
speak  of  a  " mechanism"  as  forming  an  "elemental  com- 
ponent" in  "psychic  phenomena,"  but  these  points  may  be 
passed  over.  The  power  to  learn  by  individual  experience 
is  the  evidence  which  Romanes,  Morgan,  and  Loeb  will 
accept  as  demonstrating  the  presence  of  mind  in  an  animal. 
Does  the  absence  of  proof  that  an  animal  learns  by  expe- 
rience show  that  the  animal  is  unconscious?  Romanes  is 
careful  to  answer  this  question  in  the  negative.  "  Because  a 
lowly  organized  animal,"  he  says,  "does  not  learn  by  its  own 
individual  experience,  we  may  not  therefore  conclude  that  in 
performing  its  natural  or  ancestral  adaptations  to  appro- 
priate stimuli,  consciousness,  or  the  mind  element,  is 
wholly  absent;  we  can  only  say  that  this  element,  if 
present,  reveals  no  evidence  of  the  fact"  (641,  p.  3).  Loeb, 
on  the  other  hand,  wrote  as  if  absence  of  proof  for  conscious- 
ness amounted  to  disproof,  evidently  relying  on  the  principle 
of  parsimony,  that  no  unnecessary  assumptions  should  be 
admitted.  "Our  criterion,"  he  remarked,  "puts  an  end  to 
the  metaphysical  ideas  that  all  matter,  and  hence  the  whole 
animal  world,  possesses  consciousness"  (429,  p.  13).  If 
learning  by  experience  be  really  a  satisfactory  proof  of 
mind,  then  its  absence  in  certain  animals  would  indeed 
prevent  the  positive  assertion  that  all  animals  are  con- 
scious ;  but  it  could  not  abolish  the  possibility  that  they 
might  be.  Such  a  possibility  might,  however,  be  of  no 
more  scientific  interest  than  any  one  of  a  million  wild 
possibilities  that  science  cannot  spare  time  to  disprove. 
But  we  shall  find  that  learning  by  experience,  taken  by 
itself,  is  too  indefinite  a  concept  to  be  of  much  service,  and 


32  The  Animal  Mind 

that  when  defined,  it  is  inadequate  to  bear  the  whole  weight 
of  proving  consciousness  in  animals.  Such  being  the  case, 
the  possibility  that  animals  which  have  not  been  shown 
to  learn  may  yet  be  conscious  acquires  the  right  to  be 
reckoned  with. 

The  first  point  that  strikes  us  in  examining  the  proposed 
test  is  that  the  learning  by  experience  must  not  be  too  slow, 
or  we  can  find  parallels  for  it  in  the  inanimate  world.  An 
animal  may  be  said  to  have  learned  by  experience  if  it  be- 
haves differently  to  a  stimulus  because  of  preceding  stimuli. 
But  it  is  one  thing  to  have  behavior  altered  by  a  single  pre- 
ceding stimulus,  and  another  to  have  it  altered  by  two  hun- 
dred repetitions  of  a  stimulus.  The  wood  of  a  violin  reacts 
differently  to  the  vibrations  of  the  strings  after  it  has  "expe- 
rienced "  them  for  ten  years ;  the  molecules  of  the  wood  have 
gradually  taken  on  an  altered  arrangement.  A  steel  rail  re- 
acts differently  to  the  pounding  of  wheels  after  that  process 
has  been  long  continued;  it  may  snap  under  the  strain. 
Shall  we  say  that  the  violin  and  the  rail  have  learned  by 
individual  experience  ?  If  the  obvious  retort  be  made  that 
it  is  only  in  living  creatures  that  learning  by  experience 
should  be  taken  as  evidence  of  mind,  let  us  take  an  exam- 
ple from  living  creatures.  When  a  blacksmith  has  been 
practising  his  trade  for  a  year,  the  reactions  of  his  muscles 
are  different  from  what  they  were  at  the  outset.  But  this 
difference  is  not  merely  a  matter  of  more  accurate  sense- 
discrimination,  a  better  " placing"  of  attention  and  the 
like ;  there  have  been  going  on  within  the  structure  of  his 
muscles  changes  which  have  increased  their  efficiency, 
and  with  which  consciousness  has  had  nothing  to  do. 
These  changes  have  been  extremely  slow  compared  to  the 
learning  which  does  involve  consciousness.  In  one  or  two 
lessons  the  apprentice  learned  what  he  was  to  do ;  but  only 


The  Evidence  of  Mind  33 

very  gradually  have  his  muscles  acquired  the  strength  to 
do  it  as  it  should  be  done.  Now  among  the  lower  animal 
forms  we  sometimes  meet  with  learning  by  experience  that 
is  very  slow;  that  requires  a  hundred  or  more  repetitions 
of  the  stimulus  before  the  new  reaction  is  acquired.  In 
such  a  case  we  can  find  analogical  reasons  for  suspecting 
that  a  gradual  change  in  the  tissues  of  the  body  has  taken 
place,  of  the  sort  which,  like  the  attuning  of  the  violin  wood 
or  the  slow  development  of  a  muscle,  have  no  conscious 
accompaniment . 

We  must  then  ask  the  question :  What  kind  of  learning 
by  experience  never,  so  far  as  we  know,  occurs  unconsciously  ? 
Suppose  a  human  being  shut  up  in  a  room  from  which  he  can 
escape  only  by  working  a  combination  lock.  As  we  shall  see 
later,  this  is  one  of  the  methods  by  which  the  learning  power 
of  animals  has  been  tested.  The  man,  after  prolonged 
•investigation,  hits  upon  the  right  combination  and  gets  out. 
Suppose  that  he  later  finds  himself  again  in  the  same  pre- 
dicament, and  that  without  hesitation  or  fumbling  he  opens 
the  lock  at  once,  and  performs  the  feat  again  and  again,  to 
show  that  it  was  not  a  lucky  accident.  But  one  interpreta- 
tion of  such  behavior  is  possible.  We  know  from  our  own 
experience  that  the  man  could  not  have  worked  the  lock  the 
second  time  he  saw  it,  unless  he  consciously  remembered  the 
movements  he  made  the  first  time ;  that  is,  unless  he  had  in 
mind  some  kind  of  idea  as  a  guide.  Here,  at  least,  there  can 
have  been  no  change  in  the  structure  of  the  muscles,  for  such 
changes  are  gradual ;  the  change  must  have  taken  place  in 
the  most  easily  alterable  portion  of  the  organism,  the  ner- 
vous system ;  and  further,  it  must  have  taken  place  in  the 
most  unstable  and  variable  part  of  the  nervous  system,  the 
higher  cortical  centres  whose  activity  is  accompanied  by 
consciousness.  In  other  words,  we  may  be  practically 


34  The  Animal  Mind 

assured  that  consciousness  accompanies  learning  only  when 
the  learning  is  so  rapid  as  to  show  that  the  effects  of  pre- 
vious experience  are  recalled  in  the  guise  of  an  idea  or 
mental  image  of  some  sort.  But  does  even  the  most  rapid 
learning  possible  assure  us  of  the  presence  of  an  idea  in  the 
mind  of  a  lower  animal  ?  Where  the  motive,  the  beneficial 
or  harmful  consequence  of  action,  is  very  strong,  may  not 
a  single  experience  suffice  to  modify  action  without  being 
revived  in  idea?  Moreover,  animals  as  high  in  the  scale 
as  dogs  and  cats  learn  to  solve  problems  analogous  to  that 
of  the  combination  lock  so  slowly  that  we  cannot  infer  the 
presence  of  ideas.  Are  we  then  to  conclude  that  these 
animals  are  unconscious,  or  that  there  is  absolutely  no 
reason  for  supposing  them  possessed  of  consciousness? 
Yerkes  has  criticised  the  " learning  by  experience"  criterion 
by  pointing  out  that  "no  organism  .  .  .  has  thus  far 
been  proved  incapable  of  profiting  by  experience."  It  is  a 
question  rather  of  the  rapidity  and  of  the  kind  of  learning 
involved.  "The  fact  that  the  crayfish  need  a  hundred  or 
more  experiences  for  the  learning  of  a  type  of  reaction  that 
the  frog  would  learn  with  twenty  experiences,  the  dog  with 
five,  say,  and  the  human  subject  with  perhaps  a  single 
experience,  is  indicative  of  the  fundamental  difficulty  in 
the  use  of  this  sign"  (814).  Nagel  has  pointed  out  that 
Loeb,  in  asserting  "associative  memory"  as  the  criterion  of 
consciousness,  offers  no  evidence  for  his  statement  (524). 
The  fact  is  that  while  proof  of  the  existence  of  mind  can  be 
derived  from  animal  learning  by  experience  only  if  the 
learning  is  very  rapid,  other  evidence,  equally  valid  on  the 
principle  of  analogy,  makes  it  highly  improbable  that  all 
animals  which  learn  too  slowly  to  evince  the  presence  of  ideas 
are  therefore  unconscious.  This  evidence  is  of  a  morphologi- 
cal character. 


The  Evidence  of  Mind  35 

§  7.  Inferring  Mind  from  Structure 

Both  Yerkes  and  Lukas  urge  that  the  resemblance  of  an 
animal's  nervous  system  and  sense  organs  to  those  of  human 
beings  ought  to  be  taken  into  consideration  in  deciding 
whether  the  animal  is  conscious  or  not.  Lukas  suggests 
that  the  criteria  of  consciousness  should  be  grouped  under 
three  heads:  morphological,  including  the  structure  of 
the  brain  and  sense-organs,  physiological,  and  teleological. 
Under  the  second  rubric  he  maintains  that  "individual 
purposiveness "  is  characteristic  of  the  movements  from 
which  consciousness  may  be  inferred ;  that  individual 
purposiveness  pertains  only  to  voluntary  acts,  and  that 
voluntary  acts  and  acts  "  which  are  preceded  by  the 
intention  to  perform  a  definite  movement,  hence  by  the 
idea  of  this  movement."  We  have  reached  the  same  con- 
clusion in  the  preceding  paragraph.  The  third  test  of  the 
presence  of  consciousness,  the  teleological  test,  rests  on  the 
consideration:  "What  significance  for  the  organism  may 
be  possessed  by  the  production  of  a  conscious  effect  by 
certain  stimuli?"  (445).  This  test,  however,  being  of  a 
purely  a  priori  character,  would  seem  to  be  distinctly  less 
valuable  than  the  others. 

Yerkes  proposes  "the  following  six  criteria  in  what 
seems  to  me  in  general  the  order  of  increasing  impor- 
tance. The  functional  signs  are  of  greater  value  as  a 
rule  than  the  structural;  and  within  each  of  the  cate- 
gories the  particular  sign  is  usually  of  more  value  than 
the  general.  In  certain  cases,  however,  it  might  be  main- 
tained that  neural  specialization  is  of  greater  importance 
than  modifiability. 

I.   Structural  Criteria. 

i.   General  form  of  organism  (Organization). 


36  The  Animal  Mind 

2.  Nervous  system  (Neural  organization). 

3.  Specialization  in   the  nervous   system    (Neural 

specialization). 
II.   Functional  Criteria. 

1.  General  form  of  reaction  (Discrimination). 

2.  Modifiability  of  reaction  (Docility). 

3.  Variability  of  reaction  (Initiative)"  (814). 

The  terms  " discrimination,"  " docility,"  and  "initiative" 
in  this  connection  are  borrowed  from  Royce's  "  Outlines  of 
Psychology'7  (649). 

If  resemblance  of  nervous  and  sense-organ  structure  to  the 
human  type  is  to  be  taken  along  with  rapid  learning  as  co- 
ordinate evidence  of  consciousness,  it  is  clear  that  here  also 
we  have  to  deal  with  a  matter  of  degree.  The  structure  of 
the  lower  animals  differs  increasingly  from  our  own  as  we  go 
down  the  scale.  At  what  degree  of  difference  shall  we  draw 
the  line  and  say  that  the  animals  above  it  may  be  conscious, 
but  that  those  below  it  cannot  be  ?  No  one  could  possibly 
establish  such  a  line.  The  truth  of  the  whole  matter  seems 
to  be  this :  We  can  say  neither  what  amount  of  resemblance  in 
structure  to  human  beings,  nor  what  speed  of  learning^  consti- 
tutes a  definite  mark  distinguishing  animals  with  minds  from 
those  without  minds}  unless  we  are  prepared  to  assert  that 
only  animals  which  learn  so  fast  that  they  must  have  memory 
ideas  possess  mind  at  all.  And  this  would  conflict  with 
the  argument  from  structure.  For  example,  there  is  no 
good  experimental  evidence  that  cats  possess  ideas,  yet 
there  is  enough  analogy  between  their  nervous  systems 
and  our  own  to  make  it  improbable  that  consciousness, 
so  complex  and  highly  developed  in  us,  is  in  them  wholly 
lacking.  We  know  not  where  consciousness  begins  in 
the  animal  world.  We  know  where  it  surely  resides  — 
in  ourselves;  we  know  where  it  exists  beyond  a  reason- 


The  Evidence  of  Mind  37 

able  doubt  —  in  those  animals  of  structure  resembling 
ours  which  rapidly  adapt  themselves  to  the  lessons  of 
experience.  Beyond  this  point,  for  all  we  know,  it  may 
exist  in  simpler  and  simpler  forms  until  we  reach  the  very 
lowest  of  living  beings. 


CHAPTER  III 

THE  MIND  OF  THE  SIMPLEST  ANIMALS 
§  8.   The  Structure  and  Behavior  of  Amceba 

WE  have  seen  in  the  last  chapter  that  no  one  can  prove 
the  absence  of  consciousness  in  even  the  simplest  forms  of 
living  beings.  It  is  therefore  perfectly  allowable  to  spec- 
ulate as  to  what  may  be  the  nature  of  such  consciousness, 
provided  that  the  primitive  organisms  concerned  possess  it. 
Perfectly  allowable,  yet  also  perfectly  useless,  many  authori- 
ties would  argue ;  the  remoteness  of  the  creatures  from  our- 
selves in  structure  and  behavior  renders  theorizing  about 
their  conscious  experience,  which  is  probably  non-existent 
and  certainly  unimaginable  in  any  definite  terms  by  us,  the 
idlest  form  of  mental  exercise. 

Undeniably  the  formation  of  a  positive  notion  regarding 
the  character  and  content  of  psychic  states  in  the  mind,  say 
of  an  Amceba,  is  next  door  to  an  impossibility.  Yet  it  may 
not  be  wholly  a  waste  of  time  if  we  spend  a  few  pages  in  the 
attempt  to  discover  wherein  the  simplest  type  of  mind,  sup- 
posing it  to  be  that  belonging  to  the  simplest  type  of  animal, 
necessarily  differs  from  our  own.  Some  light,  perhaps,  may 
be  cast  upon  the  growth  of  mental  life  in  complexity  if  we 
try  to  make  clear  to  ourselves  what  primitive  consciousness 
is  not,  though  we  may  not  be  able  to  find  in  our  own  experi- 
ence any  elements  that  shall  properly  represent  what  it  is. 

The  first  need  is  evidently  information  about  the  structure 
and  the  behavior  of  a  primitive  animal.  For  this  purpose 

38 


The  Mind  of  the  Simplest  Animals  39 

the  Amoeba  presents  itself  as  a  good  subject.  Structurally, 
it  consists  of  a  single  cell,  as  do  all  the  Protozoa,  the  lowest 
group  of  animals ;  it  is  so  small  that  it  can  be  studied  only 
through  the  microscope;  its  form,  at  least  that  of  Amceba 
proteus,  the  most  typical  species,  is  irregular  and  constantly 
changing  in  locomotion  or  in  response  to  stimulation. 
While  the  internal  substance  of  its  body  shows  a  certain 
amount  of  differentiation,  there  is  no  trace  whatever  of 
special  modifications  that  might  be  supposed  to  serve  for  the 
conduction  of  stimuli  to  different  parts  of  the  body,  and 
thus  represent  the  prototype  of  a  nervous  system.  Nor 
have  any  structures  been  found  that  could  conceivably  be 
used  for  the  special  reception  of  stimuli ;  that  is,  there  are 
no  sense  organs.  So  far  as  the  anatomy  of  the  animal  is 
concerned,  then,  it  differs  so  widely  from  our  own  that  we 
could  only  conclude  from  it  the  absence  of  all  those  features 
which  our  conscious  experience  involves. 

Turning  from  structure  to  behavior,  we  find  the  external 
activities  of  Amceba,  that  is,  those  not  confined  to  the  inner 
processes  of  its  cell  body,  to  be  superficially,  at  least, 
divisible  into  two  classes:  movements  of  locomotion  and 
responses  to  stimulation.  Amceba,  though  a  water-dwell- 
ing animal,  is  not  a  free-swimming  one,  but  moves  by  crawl- 
ing on  a  solid  body.  This  method  of  locomotion  involves 
in  Amceba  proteus  changes  of  form  on  the  animal's  part, 
projections,  called  pseudopodia,  being  sent  out  in  advance  of 
the  movement  of  the  whole  body.  The  protoplasm  of  the 
body  shows  in  this  process  certain  flowing  movements  which 
are  differently  described  by  different  observers,  and  doubt- 
less vary  in  different  species :  thus  Rhumbler  finds  that  the 
protoplasmic  currents  move  backward  along  the  sides  of 
the  animal  and  forward  through  the  middle  in  a  way  quite 
comparable  to  the  behavior  of  currents  in  a  drop  of  any 


40  The  Animal  Mind 

fluid  where  the  tension  of  the  surface  is  diminished  in  front, 
i.e.,  at  the  point  toward  which  the  drop,  in  consequence  of 
the  diminished  tension  there,  rolls.  Such  movements, 
Rhumbler  shows,  can  be  reproduced  by  placing,  say,  a 
drop  of  clove  oil  under  the  proper  conditions  of  surface 
tension  (632,  633).  Jennings,  on  the  other  hand,  has 
observed,  at  least  in  certain  species  of  Amoeba,  that  the 
protoplasmic  currents  are  all  forward  in  direction,  the 
movement  being  really  one  of  rolling,  complicated  by  the  at- 
tachment of  the  lower  part  of  the  body  to  the  solid  object 
on  which  the  animal  crawls.  Mechanical  conditions  of 
surface  tension  would  not  account  for  such  currents  (371, 
373,  378).  Bellinger  rejects  both  the  surface  tension  and 
the  "rolling"  theories,  and  from  a  study  of  side  views  of  the 
moving  Amoeba  concludes  that  progression  occurs  through 
the  advancement  of  the  front  end  freely  through  the  water 
and  its  subsequent  attachment,  the  rest  of  the  body  follow- 
ing through  active  contraction  brought  about  by  a  con- 
tractile substance  (181).  The  problem  is  of  great  interest 
to  the  student  of  vital  phenomena,  but  its  bearing  on  the 
question  of  mind  in  the  Amoeba  is  so  obscure  that  we 
need  not  consider  it  further,  but  may  pass  at  once  to  the 
study  of  the  animal's  reactions  to  special  stimulation. 

These  are,  according  to  Jennings  (373,  378),  the  foremost 
authority  on  the  behavior  of  the  lowest  organisms,  three  in 
number ;  namely,  the  negative,  the  positive,  and  the  food- 
taking  reactions.  First,  if  an  Amoeba  comes  into  strong 
contact  with  a  solid  obstacle  in  its  movements,  or  if  a 
solution  of  different  composition  from  the  water  in  which  it 
lives  strikes  against  it,  or  if  one  side  of  it  is  heated,  the 
animal  responds  by  contracting  the  part  stimulated,  re- 
leasing it  from  the  substratum,  and  moving  in  another 
direction,  usually  one  forming  only  a  small  angle  with  the 


The  Mind  of  the  Simplest  Animals 


preceding  one.  If  the  whole  of  one  side  or  end  receives  a 
strong  stimulus,  if  light  falls  on  one  side,  or  an  electric 
current  is  passed  through  the  water,  the  side  stimulated 
-  in  the  case  of  the  electric  current,  the  side  toward  the 
positive  pole  —  contracts  as  a  whole,  and  the  movement 
takes  place  in  the  opposite  di- 
rection. These  phenomena 
constitute  the  negative  reaction 
(Fig.  i). 

Secondly,  the  reaction  to 
solid  bodies  sometimes  takes  a 
positive  form.  In  this  case  a 
pseudopodium  is  pushed  for- 
ward in  the  direction  of  the 
stimulus,  and  the  animal  moves 
toward  the  solid.  As  the  neg- 
ative reaction  serves  the  pur-  FlG-  *•  —  Negative  reaction  of 

r  .  , .  ,  ,  Amoeba  to  stimulation  by  a  glass 

pose   Of    avoiding    obstacles,    SO  rod>    fl>  Application  of  the  stim- 

the   positive   reaction    is   Useful  ulus.      6.  Change     of     direction 

in  Securing  Contact  with  a  SUp-  ^--ement.      After    Jennings 

port  on  which  to  creep,  and 

with  food.  It  seems  to  be  given  in  response  to  weak 
mechanical  stimuli,  stronger  ones  producing  the  negative 
reaction.  No  chemicals  have  been  found  to  occasion 
it,  but  weak  chemical  stimulation  very  likely  cooperates 
with  mechanical  stimulation  when  the  positive  reaction  is 
given  to  food. 

Schaeffer  (659)  has  recently  obtained  evidence  that 
Amoeba  can  give  the  positive  reaction  to  insoluble  and 
inedible  objects  before  they  come  into  contact  with  it. 
The  way  in  which  such  objects  can  act  as  stimuli  is 
still  unexplained.  It  is  possible  that  the  movement  of 
the  Amoeba  produces  water  currents  which  are  reflected 


42  The  Animal  Mind 

back  in  a  peculiar  way  by  such  particles.  He  reports 
also  (660)  that  the  positive  reaction  is  given  to  beams 
of  light  which  pass  no  nearer  than  100-150  thousandths 
of  an  inch  to  the  animal.  The  Amoeba  moves  towards  the 
beam,  but  when  it  comes  into  contact  with  it,  the  move- 
ment ceases,  and  in  some  cases  a  negative  response  occurs. 
Thirdly,  there  is  the  food-taking  reaction.  This  consists 
for  Amoeba  proteus,  according  to  Jennings,  in  the  pushing 
forward  of  a  pseudopodium  on  either  side  of  the  particle  of 


FIG.  2. —  Food-taking  reaction  of  Amoeba,     i,  2,  3,  4,  successive  stages. 
After  Jennings  (378). 

food  that  has  come  into  contact  with  the  animal;  the 
bending  over  of  the  ends  of  the  pseudopodia  so  as  to  grasp 
the  food,  while  "a  thin  sheet  of  protoplasm"  spreads  from 
the  upper  surface  of  the  animal  over  it ;  and  the  final  fusion 
of  the  ends  of  the  pseudopodia  and  the  ends  of  this  sheet, 
so  as  to  take  the  food  directly  into  the  animal's  body. 
The  reaction  may  occur  anywhere  on  the  body  surface, 
there  being  no  specialized  mouth.  It  appears  to  be  made 
only  in  response  to  edible  substances,  hence  there  is  doubt- 
less some  chemical  peculiarity  about  the  stimulus  which 
makes  it  effective  (378). 

Kepner  and  Taliaferro  (399)  find  the  food-taking  reaction 


The  Mind  of  the  Simplest  Animals  43 

more  complex  and  variable  than  Jennings's  account  de- 
scribes it  to  be.  They  observed  cases  where  only  one 
pseudopodium  was  formed,  and  cases  where  it  was  put  forth 
not  at  the  exact  point  acted  upon  by  the  stimulus.  The 
nature  of  the  reaction  varied  in  such  a  way  as  to  prevent 
the  " swallowing"  of  too  much  water  along  with  the  food: 
"the  parts  that  could  most  advantageously  respond  did 
so."  McClendon  (451  a)  has  attempted  to  apply  the  surface 
tension  theory  to  the  positive,  negative,  and  feeding  re- 
actions of  Amoeba,  suggesting  that  the  stimuli  may  exert 
an  electric  influence  whereby  the  surface  tension  at  the  point 
stimulated  becomes  less  in  the  case  of  the  positive  reactions 
and  feeding,  greater  in  the  negative  reaction.  But  such 
variations  as  those  just  described  are  difficult  to  reconcile 
with  a  surface  tension  theory.  Moreover  Mast  and  Root 
(477)  have  observed  Amoeba  crushing  its  prey  with  a 
force  far  greater  than  surface  tension  could  account  for. 
Schaeffer  (658)  suggests  that  a  chemical  discrimination  may 
occur  inside  the  Amoeba  after  substances  have  been  taken 
in,  for,  he  says,  when  carmine  grains  have  been  swallowed, 
the  Amoeba  at  once  begins  to  move  off  in  such  a  way  as  to 
bring  the  grains  to  the  hinder  part  of  the  body  where  they 
will  be  ejected.  "The  carmine  grains  are; ejected  .  .  .  be- 
cause they  are  actually  disagreeable  and  not  merely  because 
they  are  (presumably)  indigestible."  A  hungry  Amoeba, 
when  it  comes  within  100  thousandths  of  an  inch  from  an 
organism,  which  is  as  a  whole  at  rest  but  moving  certain 
portions  of  its  body,  will  begin  to  move  towards  it  and  to 
form  a  food  cup  before  actual  contact  occurs.  Probably 
the  slight  water  currents  produced  by  the  movements  of 
the  prey  act  as  the  stimulus  in  this  case.  Any  stimulus 
which  proceeds  from  a  moving  object  tends,  as  we  shall 
see,  to  be  peculiarly  effective. 


44  The  Animal  Mind 

These  three  reactions  make  up,  together  with  the  ordinary 
crawling  locomotion,  the  variety  of  the  Amoeba's  experience 
as  displayed  in  behavior,  with  the  addition  of  a  peculiar  set 
of  movements  occurring  in  the  absence  of  all  mechanical 
stimulation.  When  an  Amoeba  is  floating  in  the  water, 
through  some  chance,  unattached  to  any  solid,  "such  a 
condition,"  says  Jennings,  "is  most  unfavorable  for  its 
normal  activities ;  it  cannot  move  from  place  to  place,  and 
has  no  opportunity  to  obtain  food."  Its  mode  of  getting 
out  of  the  difficulty  is  to  send  out  "long,  slender  pseudopodia 
in  all  directions,"  until  "the  body  may  become  reduced  to 
little  more  than  a  meeting  point  for  these  pseudopodia" 
(378,  p.  8).  As  soon  as  one  of  these  "feelers"  comes  in 
contact  with  a  solid,  it  attaches  itself,  and  the  whole  animal 
following  soon  takes  up  its  normal  crawling  locomotion. 

§  9.    The  Mind  of  Amceba 

Now  what  light  does  the  behavior  of  Amceba  throw  upon 
the  nature  of  the  animal's  possible  consciousness?  The 
first  thought  which  strikes  us  in  this  connection  is  that 
the  number  of  different  sensations  occurring  in  an  Amoeba's 
mind,  if  it  has  one,  is  very  much  smaller  than  the  number 
forming  the  constituent  elements  of  our  own  experience.  We 
human  beings  have  the  power  to  discriminate  several 
thousand  different  qualities  of  color,  brightness,  tone, 
noise,  temperature,  pressure,  pain,  smell,  taste,  and  other 
sensation  classes.  Thus  the  content  of  our  consciousness 
is  capable  of  a  great  deal  of  variety.  It  is  hard  to  see  how 
more  than  three  or  four  qualitatively  different  processes 
can  enter  into  the  conscious  experience  of  an  Amceba. 
The  negative  reaction  is  given  to  all  forms  of  strong  stim- 
ulation alike,  with  the  single  exception  of  food.  We  shall 


The  Mind  of  the  Simplest  Animals  45 

in  the  following  chapter  discuss  more  fully  the  nature  of  the 
evidence  that  helps  us  to  conjecture  the  existence  of  different 
sensation  qualities  in  an  animal's  mind ;  but  it  is  clear  that 
where  an  animal  so  simple  in  its  structure  as  the  Amoeba 
makes  no  difference  in  its  reactions  to  various  stimuli,  there 
can  be  no  reason  for  supposing  that  if  it  is  conscious,  it  is 
aware  of  them  as  different.  The  reaction  to  edible  sub- 
stances is,  however,  unlike  that  to  other  stimulations.  The 
peculiarity  of  edible  substances  which  occasions  this  differ- 
ence must  be  a  chemical  one.  In  our  own  case,  the  classes 
of  sensation  which  result  from  the  chemical  pecularities  of 
food  substances  are  smell  and  taste ;  evidently  to  a  water- 
dwelling  animal  smell  and  taste  would  be  practically  indis- 
tinguishable. We  may  say,  then,  that  supposing  conscious- 
ness to  exist  in  so  primitive  an  animal  as  the  Amoeba,  we 
have  evidence  for  the  appearance  in  it  of  a  specific  sensation 
quality  representing  the  chemical  or  food  sense,  and  standing 
for  the  whole  class  of  sensations  resulting  from  our  own 
organs  of  smell  and  taste.  The  significance  of  the  positive 
reaction  is  harder  to  determine.  It  seems  to  be  given  in  re- 
sponse not  to  a  special  kind  of  stimulus,  but  to  a  mechanical 
or  food  stimulus  of  slight  intensity.  In  our  own  experience, 
we  do  not  have  stimuli  of  different  intensity  producing  sen- 
sations of  different  quality,  except  in  the  cases  of  tempera- 
ture and  visual  sensations.  We  do,  however,  find  that 
varying  the  strength  of  the  stimulus  will  produce  different 
affective  qualities;  it  is  a  familiar  fact  that  moderate 
intensities  of  stimulation  in  the  human  organism  are 
accompanied  by  pleasantness,  and  stronger  intensities  by 
unpleasantness.  The  motor  effects  of  pleasantness  and 
unpleasantness  in  ourselves  are  opposite  to  each  other  in 
character.  Pleasantness  produces  a  tonic  and  expansive 
effect  on  the  body,  unpleasantness  a  depressive  and  con- 


46  The  Animal  Mind 

tractive  effect.  In  the  Amceba,  the  positive  and  negative 
reactions  seem  to  be  opposed.  The  essential  feature  of 
the  negative  reaction  is  the  checking  of  movement  at  the 
point  stimulated ;  that  of  the  positive  reaction  is  the  reach- 
ing out  of  the  point  stimulated  in  the  direction  of  the 
stimulus.  This  much  evidence  there  is  for  saying  that 
besides  a  possible  food  sensation,  the  Amceba  may  have 
some  dim  awareness  of  affective  qualities  corresponding 
to  pleasantness  and  unpleasantness  in  ourselves.  It  should, 
however,  be  borne  in  mind  that  wide  differences  must  go 
along  with  the  correspondence.  In  us,  pleasantness 
brings  a  thrill,  a  " bodily  resonance,"  due  to  its  tonic  effect 
upon  the  circulation,  breathing,  and  muscles ;  unpleasant- 
ness has  also  its  accompaniment  of  vague  organic  sensation, 
without  which  we  can  hardly  conceive  what  it  would  be 
like.  In  an  Amceba,  it  is  clear  that  this  aspect,  as  found 
in  human  consciousness,  must  be  wholly  lacking.  Again, 
in  the  human  mind  pleasantness  and  unpleasantness  are 
connected  with  various  sensation  qualities  or  complexes; 
we  are  pleased  or  displeased  usually  "at"  something 
definite.  The  vagueness  of  the  affective  qualities  in  an 
Amoeba's  consciousness  can  only  be  remotely  suggested  by 
our  own  vague,  diffused  sense  of  bodily  well-being  or  ill- 
being;  and  this  is  undoubtedly  given  its  coloring  in  our 
case  by  the  structure  and  functioning  of  our  internal  organs. 
As  for  the  peculiar  behavior  of  an  Amceba  suspended  in 
the  water  and  deprived  of  solid  support,  the  stimulus  for 
this  must  lie  within  the  cell  body  itself.  If  any  conscious- 
ness accompanies  it,  then  the  nearest  human  analogy  to 
such  consciousness  is  to  be  found  in  organic  sensations,  and 
these,  as  has  just  been  said,  must  necessarily  be  in  the  human 
mind  wholly  different  in  quality  from  anything  to  be  found 
in  an  animal  whose  structure  is  as  simple  as  the  Amoeba's. 


The  Mind  of  the  Simplest  Animals  47 

A  consequence  of  this  lack  of  qualitative  variety  in  the 
sense  experiences  of  an  Amoeba  is  a  lack  of  what  we  may 
call  complexity  of  structure  in  that  experience.  The 
number  of  stimulus  differences  which  are  in  the  human 
mind  represented  by  differences  in  the  quality  of  sensations 
is  so  great  that  at  any  given  moment  our  consciousness  of 
the  external  world  is  analyzable  into  a  large  number  of 
qualitatively  different  sensations.  At  the  present  instant 
the  reader's  consciousness  "contains,"  apart  from  the  re- 
vived effects  of  previous  stimulation,  many  distinguish- 
able sensation  elements,  visual,  auditory,  tactile,  organic, 
and  so  on.  The  Amoeba's  consciousness,  if  it  possesses 
one,  must  have  a  structure  inconceivably  simpler  than  that 
of  any  moment  of  our  own  experience. 

A  second  point  in  which  the  mind  of  an  Amoeba  must,  if 
it  exists,  differ  from  that  of  a  human  being,  consists  in  its 
entire  lack  of  mental  imagery  of  any  sort.  Not  only  has  the 
Amoeba  but  three  or  four  qualitatively  different  elements 
in  its  experience,  but  none  of  these  qualities  can  be  re- 
membered or  revived  in  the  absence  of  external  stimulation. 
How  may  we  be  sure  of  this  ?  If  our  primitive  animal  could 
revive  its  experiences  in  the  form  of  memory  images,  it  would 
give  some  evidence  of  the  influence  of  memory  in  its  be- 
havior. Indeed,  as  we  shall  learn,  it  is  possible,  in  all 
probability,  for  an  animal's  conduct  to  be  influenced  by 
its  past  experience  even  though  the  animal  be  incapable 
of  reviving  that  experience  in  the  form  of  a  memory  image. 
Therefore,  if  we  find  no  evidence  that  the  Amoeba  learns, 
or  modifies  its  behavior  as  the  result  of  past  stimulation,  we 
may  conclude  a  fortiori  that  it  does  not  have  memory  images. 

Now  it  would  be  stating  the  case  too  strongly  to  say  that 
past  stimulation  does  not  affect  the  behavior  of  Amoeba  at 
all.  In  the  first  place,  this  animal  shows,  in  common  with 


48  The  Animal  Mind 

all  other  animals,  the  power  of  "getting  used"  to  certain 
forms  of  stimulation,  so  that  on  long  continuance  they  cease 
to  provoke  reaction.  "Thus,"  Jennings  says,  "Amoebae 
react  negatively  to  tap  water  or  to  water  from  a  foreign 
culture,  but  after  transference  to  such  water  they  behave 
normally"  (378,  p.  20).  Such  cessation  of  reaction  occurs 
when  the  continued  stimulus  is  not  harmful.  In  a  sense, 
it  may  be  called  an  effect  of  experience ;  but  there  is  clearly 
no  reason  for  supposing  that  it  involves  the  revival  of 
experience  in  the  form  of  an  idea  or  image.  We  have 
parallel  phenomena  in  our  own  mental  life.  A  continued 
stimulus  ceases  to  be  "noticed,"  but  the  process  involves 
rather  the  disappearance  of  consciousness  than  the  appear- 
ance of  a  memory  image.  Jennings,  however,  is  inclined 
to  think  that  preceding  stimulation  may  modify  the 
Amoeba's  behavior  in  a  way  more  nearly  suggesting  memory 
in  a  higher  type  of  mind.  He  describes  an  interesting 
observation  to  illustrate  this.  A  large  Amoeba,  c,  had 
swallowed  a  smaller  one,  £,  but  had  left  a  small  canal 
open,  through  which  the  swallowed  one  made  efforts  to 
escape,  which  were  several  times  foiled  by  movements  on 
the  part  of  the  large  Amoeba  toward  surrounding  it  again. 
Finally  it  succeeded  in  getting  completely  out,  whereupon 
the  large  Amoeba  "reversed  its  course,  overtook  b,  engulfed 
it  completely  again,  and  started  away."  The  small  Amoeba 
contracted  into  a  ball  and  remained  quiet  until  through  the 
movements  of  the  large  one  there  chanced  to  be  but  a  thin 
layer  of  protoplasm  covering  it.  This  it  rapidly  pushed 
through,  escaped  completely,  and  was  not  pursued  by  the 
large  Amoeba  (378,  pp.  17-18),  (Fig.  3). 

Of  this  performance  Jennings  says:  "It  is  difficult  to 
conceive  each  phase  of  action  of  the  pursuer  to  be  completely 
determined  by  a  simple  present  stimulus.  For  example 


50  The  Animal  Mind 

.  .  .  after  Amoeba  b  has  escaped  completely  and  is  quite 
separate  from  Amoeba  c,  the  latter  reverses  its  course  and 
recaptures  b.  What  determines  the  behavior  of  c  at  this 
point?  If  we  can  imagine  all  the  external  physical  and 
chemical  conditions  to  remain  the  same,  with  the  two 
Amoebae  in  the  same  relative  positions,  but  suppose  at  the 
same  time  that  Amoeba  c  has  never  had  the  experience  of 
possessing  b}  —  would  its  action  be  the  same  ?  Would  it 
reverse  its  movement,  take  in  b,  then  return  on  its  former 
course  ?  One  who  sees  the  behavior  as  it  occurs  can  hardly 
resist  the  conviction  that  the  action  at  this  point  is  partly 
determined  by  the  change  in  c  due  to  the  former  possession 
of  b,  so  that  the  behavior  is  not  purely  reflex"  (378,  p.  24). 

If  it  is  true  that  an  Amoeba  which  had  not  just  "had  the 
experience  of  possessing  b"  would  not  have  reversed  its 
movement  and  gone  after  b  when  the  latter  escaped,  still 
we  cannot  think  it  possible  that  c's  movements  in  so  doing 
were  guided  by  a  memory  image  of  b.  It  may  be  supposed 
that  the  recent  stimulation  of  contact  with  b  had  left  a  part 
of  c's  protoplasm  in  a  condition  of  heightened  excitability, 
so  that  the  weak  stimulus  offered  perhaps  by  slight  water 
disturbances  due  to  5's  movements  after  escaping  produced 
a  positive  reaction,  although  under  other  circumstances  no 
reaction  would  have  been  possible.  (Compare  the  observa- 
tion of  Schaeffer,  just  quoted,  on  Amoeba's  ability  to  react 
to  objects  not  in  contact  with  it.)  In  any  case,  there  is  no 
evidence  that  Amoeba's  behavior  is  influenced  by  stimula- 
tion occurring  earlier  than  the  moments  just  preceding 
action ;  no  proof  of  the  revival  of  a  process  whose  original 
effects  have  had  time  to  die  out ;  and  it  is  upon  such  revival 
that  the  memory  images  which  play  so  much  part  in  our 
own  conscious  life  depend. 

Let  us  consider  for  a  moment  some  of  the  results  of  the 


The  Mind  of  the  Simplest  Animals  51 

absence  of  this  kind  of  material  in  the  possible  mental  pro- 
cesses of  Amoeba.  In  the  first  place,  such  a  lack  profoundly 
affects  the  character  of  the  experiences  which  the  animal 
might  be  supposed  to  receive  through  external  stimulation. 
If  we  call  the  possible  conscious  effect  of  a  mechanical  stimu- 
lus upon  the  Amoeba  a  touch  sensation,  the  term  suggests, 
naturally,  such  sensations  as  we  ourselves  experience  them. 
In  normal  human  beings  touch  sensations  are  accompanied 
by  visual  suggestions,  more  or  less  clear,  of  course,  according 
to  the  visualizing  powers  of  the  individual,  but  always  pres- 
ent in  some  degree.  Fancy,  for  example,  one  of  us  enter- 
ing a  room  in  the  dark  and  groping  about  among  the  furni- 
ture. How  constantly  visual  associations  are  brought  into 
play !  Not  once  is  a  mere  touch  impression  apprehended 
without  being  translated  into  visual  terms ;  the  forms  and 
positions  of  the  articles  encountered  are  thought  of  imme- 
diately as  they  would  appear  if  the  room  were  lighted. 
The  difficulty  we  have  in  thinking  of  a  touch  sensation  with 
no  visual  associations  illustrates  the  difference  between  our 
sense  experience  and  that  of  an  animal  incapable  of  recalling 
images  of  past  sensations. 

It  is  equally  obvious  that  in  the  absence  of  memory  ideas, 
not  only  must  the  Amoeba  lack  processes  of  imagination  and 
reasoning,  but  there  can  be  nothing  like  the  continuous  self- 
consciousness  of  a  human  being,  the  " sense"  of  personal 
identity,  which  depends  upon  the  power  to  revive  past 
experiences.  It  is  even  possible  that  the  "stream  of 
consciousness"  for  an  Amoeba  may  not  be  a  continuous 
stream  at  all.  Since  its  sensitiveness  to  changes  in  its 
environment  is  less  developed  than  that  of  a  human  being, 
and  there  are  no  trains  of  ideas  to  fill  up  possible  intervals 
between  the  occurrences  of  outside  stimulation,  the 
Amoeba's  conscious  experience  may  be  rather  a  series  of 


52  The  Animal  Mind 

11  flashes"  than  a  steady  stream.  And  for  the  Amoeba, 
again,  we  must  remember  that  even  such  a  series  would  not 
exist  as  such ;  the  perception  of  a  series  would  involve  the 
revival  of  its  past  members.  Each  moment  of  conscious- 
ness is  as  if  there  were  no  world  beyond,  before,  and 
after  it. 

Another  consequence  of  that  simplicity  of  structure  which 
results  both  from  the  rudimentary  powers  of  sensory  dis- 
crimination and  from  the  absence  of  memory  ideas  in  the 
Amoeba's  mind  is  that  there  can  be  no  distinction,  within 
a  given  mental  process,  between  that  which  is  attended  to 
and  that  which  is  not  attended  to,  between  the  focus  and 
the  margin  of  consciousness.  Given  a  consciousness  which 
at  a  certain  moment  is  composed  of  the  qualitatively  differ- 
ent elements  A,  B,  C,  and  D,  we  can  understand  what  is 
meant  by  saying  that  A  is  attended  to,  is  in  the  foreground 
of  attention,  while  B,  C,  and  D  remain  in  the  background. 
But  given,  on  the  other  hand,  a  creature  whose  conscious 
content  at  a  certain  time  consists  wholly  of  the  qualitatively 
simple  experience  A,  it  is  evident  that  attention  and  in- 
attention are  meaningless  terms.  Different  moments  of 
its  consciousness  may  differ  in  intensity;  but  attention, 
involving,  as  it  does,  clearness  rather  than  intensity,  arises 
only  when  mental  states  have  become  complex  and  possess 
detail  and  variety  within  their  structure. 


CHAPTER  IV 

SENSORY  DISCRIMINATION:  METHODS  OF  INVESTIGATION 
§  10.   Preliminary  Considerations 

ONE  of  the  most  important  points  in  which  the  human 
mind  differs  from  the  mind  of  the  lowest  animal  forms  con- 
sists, we  have  seen,  in  the  enormously  greater  number  of 
different  sensations  which  enter  into  human  experience,  as 
compared  with  the  small  number  of  sensory  discriminations 
possible  to  the  simpler  animals.  Much  of  the  experimental 
work  that  has  been  done  on  animals  has  been  directed 
toward  discovering  what  discriminations  they  make  among 
the  stimuli  acting  upon  them,  and  to  the  results  of  this 
work  we  shall  give  our  attention  in  the  next  chapters.  But 
first  we  ought  to  get  a  clearer  idea  of  just  what  kind  of 
evidence  is  needed  to  indicate  the  existence  of  a  variety  of 
sensations  in  an  animal's  mind. 

At  the  outset,  we  must  remind  ourselves  that,  in  the 
absence  of  any  satisfactory  proof  that  the  lower  animal 
forms  have  minds  at  all,  and  the  equal  absence  of  any  proof 
that  they  have  not,  all  our  conclusions  about  the  number 
and  kind  of  their  possible  sensations  must  remain  subject 
to  the  proviso  that  they  possess  consciousness.  Further, 
a  point  that  was  mentioned  in  Chapter  I  must  again  be 
emphasized.  No  evidence  of  discrimination  between  two 
stimuli  on  an  animal's  part  can  do  more  than  show  us  that 
for  the  animal  they  are  different;  just  what  the  quality 
of  the  sensation  resulting  from  each  may  be,  whether  it 

53 


54  The  Animal  Mind 

is  identical  with  any  sensation  quality  entering  into  our 
own  experience,  we  cannot  say.  The  light  rays  which 
to  us  are  red  and  blue  may  for  an  animal's  consciousness 
also  differ  from  each  other,  and  yet  if  our  experience  could 
be  exchanged  for  the  animal's,  we  might  find  in  the  latter 
nothing  like  red  and  blue  as  we  know  them. 

Thus  much  being  premised,  what  sort  of  evidence  can 
be  obtained  that  an  animal  does  discriminate  between  two 
stimuli?  Again,  as  in  considering  the  evidence  for  the 
existence  of  consciousness  in  general,  there  is  an  argument 
from  structure  and  an  argument  from  behavior. 

§  ii.   Structure  as  Evidence  of  Discrimination 

The  argument  from  structure  consists  primarily  in  the 
fact  that  an  animal  possesses  sense  organs  recognizably 
like  our  own.  If  a  creature  has  an  organ  suggesting 
strongly  the  construction  of  the  human  cochlea,  or  an 
organ  with  a  lens  and  a  membrane  composed  of  rods  and 
cones,  it  is  highly  probable  that  auditory  stimuli  in  the  one 
case  and  light  in  the  other  produce  specific  sensations.  This 
argument  from  the  morphology  of  sense  organs  is,  however, 
limited  in  two  ways.  First,  it  is  only  a  small  part  of  the 
animal  world  whose  sense  organs  resemble  ours  closely 
enough  to  make  the  analogy  safe.  And  secondly,  we  do 
not  after  all  know  very  much  about  the  relation  of  our  own 
sense-organ  structure  to  function.  We  know,  for  example, 
that  our  own  organ  with  a  lens  and  retina  gives  us  visual 
sensations,  but  we  cannot  say  with  certainty  which  struc- 
tures in  the  retina  furnish  brightness  sensations  and  which 
color  sensations,  nor  do  we  know  anything  about  the 
retinal  structures  that  underlie  different  qualities  of  color 
sensations.  We  can  say  that  sensations  of  hearing  come 


Sensory  Discrimination:  Methods  of  Investigation      55 

from  the  ear,  but  no  one  can  tell  us  how  to  judge  from  the 
structure  of  the  ear  what  range  and  fineness  of  pitch  dis- 
criminations exist  in  its  possessor's  mind.  No  investi- 
gator has  yet  succeeded  in  relating  the  different  qualities 
of  smell  and  taste  to  differences  in  the  end  organs. 


§  12.  Behavior  as  Evidence  of  Discrimination 

The  argument  from  behavior  is  as  follows :  If  an  animal 
reacts  in  a  different  way  to  two  qualitatively  unlike  stimuli, 
then,  providing  that  it  is  conscious  at  all,  it  may  be  sup- 
posed to  receive  qualitatively  unlike  sensations  from  them. 
If  it  always  reacts  in  the  same  way  to  both,  then  both  may  be 
supposed  to  be  accompanied  by  the  same  sensation  quality. 
Obviously  these  statements  need  further  discussion.  For 
one  thing,  it  may  be  urged  that  in  our  own  case  the  same 
external  reaction  is  often  made  to  stimuli  that  are  never- 
theless consciously  discriminated.  A  man  may  eat  with 
relish  and  without  observable  difference  in  behavior,  for 
example,  foods  that  yet  give  him  perfectly  distinguish- 
able smell  and  taste  sensations.  Precisely  this  objection 
holds  against  a  method  of  experimentation,  formerly  a 
good  deal  used,  which  may  be  called  the  Preference  Method 
of  testing  discrimination.  Vitus  Graber,  for  instance, 
attempted  to  find  whether  animals  belonging  to  a  variety 
of  species  could  discriminate  colors,  by  offering  them  the 
choice  of  two  compartments  illuminated  each  with  a  dif- 
ferent color.  Clearly,  if  the  animals  chose  one  compart- 
ment as  often  as  the  other,  it  would  be  rash  to  conclude 
that  the  two  lights  produced  for  them  indistinguishable 
sensation  qualities.  There  might  simply  be  the  absence 
of  any  preference,  along  with  perfect  discrimination. 
The  fact  is  that  in  all  experiments  upon  animals,  whether 


56  The  Animal  Mind 

to  determine  their  power  of  distinguishing  stimuli  or  their 
power  of  learning  by  experience,  the  first  requisite  is  to 
give  the  animal  what  we  commonly  call  a  motive.  That  is, 
the  conditions  of  the  experiment  must  be  so  arranged  that 
some  already  present  tendency  to  act,  whether  inborn  in 
the  animal  or  acquired  by  previous  experience,  shall  be 
appealed  to. 

This  is  increasingly  the  case,  the  higher  the  animal  worked 
with  stands  in  the  scale.  The  higher  animals  have  what 
might  be  called  a  large  reserve  fund  of  discriminations. 
That  is,  they  are  capable  of  making  many  more  selective 
reactions  to  stimuli  than  they  need  at  a  given  moment 
actually  to  use.  Hence  in  their  case  the  experimenter  must 
make  a  careful  adjustment  of  conditions  to  bring  out 
exactly  the  discrimination  wanted.  He  must  either  make 
the  performance  of  the  reaction  pleasant  or  its  non-per- 
formance unpleasant  to  the  animal.  A  monkey,  for  ex- 
ample, confronted  by  a  set  of  glass  tumblers  covered 
each  with  a  differently  colored  paper,  may  behave  toward 
them  all  in  precisely  the  same  way;  yet  if  food  be  put 
regularly  in  the  blue  tumbler,  whose  position  in  the  row  is 
varied,  it  becomes  worth  the  monkey's  while  to  make  use 
of  his  discriminative  powers,  and  he  may  show  by  his  dif- 
ferent behavior  toward  the  blue  tumbler  that  it  produces 
on  him  a  different  impression  from  the  others. 

With  simpler  animals  the  problem  is  less  difficult.  If 
an  animal  is  capable  only  of  a  half  dozen  different  ways 
of  responding  to  stimulation,  we  may  with  comparative 
safety  assume  that  it  has  less  opportunity  to  hold  them  in 
reserve;  and  if  such  an  animal  invariably  reacts  in  the 
same  way  to  two  different  forms  of  stimulus,  or  if  the 
variations  in  its  response  are  not  correlated  with  differ- 
ences in  the  stimulation,  it  becomes  probable  that  the  two 


Sensory  Discrimination:  Methods  of  Investigation      57 

stimuli  produce  in  its  assumed  consciousness  identical 
sensation  qualities.  Thus  it  is  not  the  number  of  stimuli 
to  which  an  animal  reacts  that  can  be  taken  as  evidence 
of  the  qualitative  variety  of  its  sensations,  but  the  number 
of  stimuli  to  which  it  gives  different  reactions.  When 
Jennings,  for  instance,  says  that  Amoeba  "  reacts  to  all 
classes  of  stimuli  to  which  higher  animals  react"  (378, 
p.  19),  we  cannot  conclude  that  it  possesses  all  classes  of 
sensations  that  higher  animals  possess,  for  its  reactions 
to  these  different  stimuli  are  but  little  varied  according  to 
the  kind  of  stimulus.1 

An  ingenious  way  of  getting  evidence  from  behavior  is 
the  salivary  reflex  method  devised  by  the  Russian  physi- 
ologist Pawlow  (830).  The  salivary  ducts  of  the  dog, 
which  lie  near  the  surface,  are  operated  on  so  that  the 
saliva  can  be  discharged  into  a  graduated  tube.  As  is 
well  known,  the  sight  or  smell  of  food  increases  the  flow  of 
saliva.  Now  when  any  other  stimulus,  such  as  a  sound, 
regularly  accompanies  the  sight  or  smell  of  food,  this  stimu- 
lus, originally  without  effect  on  the  salivary  flow,  comes 
to  increase  the  flow  even  in  the  absence  of  food.  If,  now, 
the  stimulus  that  has  thus  acquired  the  power  to  affect 
the  salivary  flow  is  given  in  irregular  alternation  with  an- 
other stimulus  differing  slightly  from  it,  and  the  other 
stimulus  is  found  not  to  affect  the  flow  of  saliva,  then  the 
inference  can  be  drawn  that  sensory  discrimination  between 
these  two  stimuli  is  possible  for  the  animal.  It  is  main- 
tained by  some  investigators  that  when  sensory  discrimi- 
nation can  be  studied  through  such  simple  types  of  behavior 

1  One  of  many  reasons  for  the  unsatisfactoriness  of  an  article  by  A. 
Olzelt-Newin,  entitled  "Beobachtungen  liber  das  Leben  der  Protozoen" 
(529),  lies  in  the  author's  uncritical  acceptance  of  the  hypothesis  that  re- 
action to  a  special  kind  of  stimulus  means  a  special  kind  of  sensation. 


58  The  Animal  Mind 

as  the  salivary  and  other  reflexes,  there  is  less  chance  of 
misinterpretation  than  when  more  complicated  choice  pro- 
cesses are  involved. 

In  all  experiments  where  behavior  alone  is  the  basis  of 
inference  regarding  sensory  discrimination,  we  need  to  take 
the  utmost  care  that  the  animal  is  really  responding  to  the 
stimuli,  and  not  to  some  other  accidental  cue.  Thus  a 
dog  in  the  Harvard  laboratory  was  apparently  discrimi- 
nating accurately  between  two  lighted  areas  of  different 
size,  but  events  proved  that  he  was  actually  responding  to 
slight  pulls  given  by  the  experimenter  on  the  leash  that 
held  him.  He  failed  wholly  when  he  was  taken  off  the 
leash.  Nowadays  the  careful  experimenter  always  re- 
mains out  of  sight  and  hearing  of  the  animal  tested,  and 
is  not  in  contact  with  it  in  any  way. 

§  13.   Evidence  from  Structure  and  Behavior  Combined 

As  a  matter  of  fact,  the  argument  from  structure  needs 
confirmatory  evidence  from  behavior.  For  clearly  the 
mere  presence  of  a  sense  organ  bearing  sufficient  likeness 
to  our  own  to  admit  of  conjecturing  its  function  would  be 
of  no  value  as  proof  unless  it  were  shown  that  the  sense 
organ  actually  functioned.  In  order  to  do  this,  it  would 
be  necessary  to  show  that  the  animal  reacted  to  the  stimulus 
conjectured  as  appropriate  to  the  sense  organ,  and  that 
removal  of  the  organ  profoundly  modified  the  reaction. 
Thus  we  shall  find  that  many  experiments  to  test  sensory 
discrimination  have  been  made  by  the  method  of  extir- 
pating a  sense  organ  and  studying  the  effect  on  behavior. 
The  method  has  many  disadvantages,  the  chief  of  which 
lies  in  the  fact  that  it  is  hard  to  say  which  disturbances 
in  behavior  are  due  actually  to  the  loss  of  the  organ  and 


Sensory  Discrimination:  Methods  of  Investigation      59 

which  to  the  more  widespread  effects  of  the  operation. 
Yet  this  much  may  be  said  for  the  combination  of  proof 
from  structure  and  behavior  involved  in  the  Method  of 
Extirpation,  if  we  may  so  call  it :  where  an  animal  reacts 
to  a  certain  stimulus,  for  instance  light,  when  a  sense  organ 
is  intact,  and  fails  to  react  to  light,  though  otherwise  nor- 
mal, when  the  organ  is  removed,  there  arises  a  possibility 
that  light  may  produce  in  the  animal's  consciousness  a 
specific  sensation  quality,  even  although  the  animal  ordinarily 
reacts  to  light  in  a  manner  indistinguishable  from  that  of  its 
responses  to  other  stimuli.  Though  light  and  mechanical 
stimulation,  for  example,  both  ordinarily  produce  a  nega- 
tive reaction,  yet  if  light  brings  about  its  effect  only  through 
the  medium  of  a  specialized  structure  with  which  mechanical 
stimuli  are  not  concerned,  then  along  with  the  probable 
unpleasantness  accompanying  the  negative  reaction  there 
may  go  a  quality  peculiar  to  the  functioning  of  that  special 
structure. 

Another  mode  of  combining  evidence  from  structure 
with  evidence  from  behavior  is  by  the  use  of  localized  stimuli. 
If  an  animal  gives  a  response,  which  in  itself  may  have 
nothing  to  mark  it  off  from  responses  to  other  stimuli, 
when  a  special  kind  of  stimulation  is  applied  to  certain 
regions  of  the  body,  and  only  then,  while  the  other  stimuli 
produce  better  reactions  when  applied  elsewhere,  then 
the  suggestion  is  given  that  different  sense  organs  are 
involved,  and  the  same  possibility  arises  of  different  sensa- 
tion qualities. 

Two  other  forms  of  evidence  whereby  from  behavior  a 
differentiation  of  sensory  structures  can  be  argued,  and 
from  differentiation  of  sensory  structures  possible  differ- 
ences of  sensation  quality,  may  be  mentioned.  The  first 
of  these  consists  in  showing  that  reactions  to  different 


60  The  Animal  Mind 

stimuli  may  be  independently  fatigued.  The  natural  in- 
ference is  that  a  specific  nervous  apparatus  belongs  to 
each  stimulus.  The  second  lies  in  demonstrating  that  the 
reactions  to  different  stimuli  occur  with  different  degrees 
of  rapidity.  If  there  is  a  marked  difference  in  the  reaction 
times  of  an  animal  to  different  forms  of  stimulation,  each, 
again,  may  be  supposed  to  affect  its  own  nervous  path- 
way. A  modification  of  this  method  consists  in  noting 
the  influence  of  a  stimulus  upon  the  time  of  reaction  to 
another  nearly  simultaneous  stimulus.  If  such  an  influ- 
ence can  be  shown,  it  is  evident  that  the  force  producing 
it  has  some  effect  on  the  nervous  system.  By  combining 
this  method  with  that  of  extirpating  a  sensory  structure, 
indications  may  be  obtained  that  the  nervous  effect  of  the 
auxiliary  stimulus  is  dependent  on  a  definite  receptive 
apparatus,  and  hence  is  probably  accompanied  by  a  special 
sensation.  This  method  was  used  by  Yerkes  to  demon- 
strate hearing  in  frogs  (813). 

One  further  consideration  offers  itself  to  the  student  of 
animal  responses  to  stimulation.  It  has  been  the  special 
endeavor  of  Jennings  to  point  out  the  fact  that  these  re- 
sponses, instead  of  being  wholly  accounted  for  by  the 
characteristics  of  the  stimulus,  are  determined  in  part  by 
the  internal,  physiological  condition  of  the  animal  (378). 
We  shall  therefore  note  often  in  the  course  of  the  follow- 
ing pages  cases  where  difference  of  reaction  is  due  to  in- 
ternal rather  than  to  external  causes. 


§14.   Evidence  for   Discrimination  of   Certain    "Lower" 
Sensation  Classes 

Bearing  all  these  points  in  mind,  let  us  proceed  to  survey 
the  evidence  for  variety  in  the  sensations  of  animals.     In 


Sensory  Discrimination:  Methods  of  Investigation      61 

the  lowest  forms,  such  evidence  must  be  derived  entirely 
from  behavior.  That  from  the  presence  of  a  sense  organ 
is  almost  wholly  lacking.  And  although  various  stimuli, 
as  we  have  seen,  produce  reactions  in  Amoeba,  yet  there 
is  only  one  case  where  these  reactions  are  strikingly  dif- 
ferent according  to  the  quality  of  the  stimulus  applied. 
This  instance  consists  in  the  distinction  between  food- 
taking  reactions,  given  to  edible  substances,  and  the 
responses  to  mechanical  stimulation.  The  sense  of  touch, 
undoubtedly,  must  play  a  part  in  the  mental  life  of  the 
lowest  animals  that  have  consciousness  at  all.  But  the 
earliest  distinction  between  a  touch  quality  and  a  quality 
that  is  other  than  touch  seems  to  occur  when  food  sensa- 
tion and  contact  sensation  are  differentiated.  It  is  possible 
that  warmth  and  cold  also  appear  as  distinct  sensa- 
tion qualities  in  the  experience  of  low  forms  of  animals, 
but  we  have  little  real  evidence  of  the  fact.  No  organs  of 
temperature  sensation  are  definitely  known  even  in  hu- 
man beings.  And  the  responses  of  low  animals  to  thermal 
stimulation  are  not  specialized.  They  consist  usually  of 
negative  reactions,  given  when  the  animal  is  subjected  to 
a  temperature  either  above  or  below,  but  especially  above, 
the  "optimum";  and  these  reactions  are  not- different 
from  the  ordinary  negative  type,  suggesting  unpleasantness 
rather  than  a  specific  sensation  quality.  In  some  cases 
the  sensibility  to  thermal  stimulation  has  been  found  to 
be  differently  distributed  from  that  to  other  classes  of 
stimuli.  But  in  any  case,  sensations  of  warmth  and 
cold  are  probably  in  no  member  of  the  animal  kingdom 
differentiated  into  any  greater  number  of  qualitatively  dis- 
tinct sensations. 

The  sense  of  touch,  also,  shows  but  little  internal  dif- 
ferentiation.    Its  importance,  so  far  as  we  can  judge,  is 


62  The  Animal  Mind 

rather  on  the  spatial  than  on  the  qualitative  side.  The 
sense  quality  of  pain  we  naturally  think  of  as  the  ac- 
companiment of  the  negative  reaction  in  its  more  violent 
forms,  given  to  a  stimulus  that  is  injuring  the  organism. 
Organic  and  kinaesthetic  sensations  are  hard  to  trace  in 
the  lower  animals;  for  animals  whose  structure  differs 
widely  from  our  own,  the  qualities  of  these  two  classes 
must  remain  beyond  the  power  of  our  imagination.  That 
differences  in  physiological  condition  such  as  are  produced 
by  hunger,  satiety,  or  fatigue  involve  differences  of  ac- 
companying organic  sensation  in  the  consciousness  of  the 
animal  manifesting  them  is  possible.  Kinaesthetic  sen- 
sations, as  we  shall  see,  are  apparently  concerned  in  the 
processes  whereby  many  animals  have  learned  to  traverse 
a  labyrinth  path. 

The  three  classes  of  sensation  whose  existence  in  the 
animal  mind  can  be  most  satisfactorily  traced  are  the 
chemical  sense,  under  which  smell  and  taste  belong,  the 
sense  of  hearing,  and  the  sense  of  sight.  To  the  study  of 
these  the  following  chapters  will  be  devoted.  Since  the 
manifestations  of  the  chemical  sense  in  the  lowest  forms  of 
animals  consist  chiefly  in  a  differentiation  of  response  to 
food  and  to  mechanical  stimulation,  the  contact  sense  or 
sense  of  touch  will,  in  discussing  these  forms,  be  considered 
along  with  the  chemical  sense. 


CHAPTER  V 

SENSORY  DISCRIMINATION:  THE  CHEMICAL  SENSE 
§  15.    The  Chemical  Sense  in  Protozoa 

WE  have  already  seen  that  the  most  primitive  type  of 
protozoon,  Amoeba  proteus,  discriminates  between  edible 
and  inedible  substances.  While  it  will  sometimes  '  swal- 
low' inedible  particles  such  as  grains  of  carmine,  it  takes 
immediate  measures  to  get  rid  of  them,  measures  too  prompt 
to  be  the  result  of  an  actual  attempt  at  digestion,  and 
hence  properly  to  be  regarded  as  the  effect  of  a  chemical 
or  food  sense.  Many  other  members  of  the  lowest  division 
of  the  animal  kingdom,  the  Protozoa,  have  a  structure  and 
behavior  decidedly  more  complicated  than  those  of  Amoeba. 
There  is  a  large  group  of  single-celled  animals  called  Ciliata, 
from  the  fact  that  their  bodies  are  covered  with  little  hair- 
like  protoplasmic  filaments  or  cilia  which  serve  as  organs 
of  locomotion  by  acting  like  tiny  oars.  A  common  repre- 
sentative of  the  group  is  Paramecium.  The  structure  of 
this  animal  is  distinctly  more  specialized  than  that  of 
Amoeba.  Not  only  are  the  cilia  modified  locomotory 
structures,  but  there  is  a  definite  region  for  food-taking. 
A  groove  extends  obliquely  down  one  side  of  the  body, 
terminating  at  its  lower  end  in  a  mouth.  The  cilia  along 
this  oral  groove  beat  with  especial  vigor  and  create  currents 
which  sweep  food  particles  to  the  mouth.  Paramecium 
swims  rapidly  through  the  water  with  a  spiral  motion  of 
its  body,  due  to  the  facts  that  the  aboral  cilia  beat  more 

63 


64  The  Animal  Mind 

strongly  than  the  rest,  and  that  the  animal  compensates 
for  the  turning  thus  occasioned  by  turning  on  its  long  axis. 
Its  reactions  to  stimulation  Jennings  has  shown  to  be  only 
two  in  number.  First,  there  is  a  very  definite  avoiding  or 
negative  reaction.  This  is  given  in  response  to  decided 
mechanical  stimulation  at  the  anterior  end,  as  when  the 
animal  swims  rapidly  against  an  obstacle,  and  also  in 
response  to  chemical  stimulation,  to  strong  ultra-violet 
rays  (299),  and  to  temperatures  above  or  below  a  certain 
middle  region  called  in  this  case,  as  in  analogous  cases  with 
other  animals,  the  optimum.  For  Paramecium  it  lies  be- 
tween 24°  and  28°  C.  The  negative  reaction  consists, 
according  to  Jennings,  of  the  following  process  :  the  animal 
darts  backward,  reversing  the  beat  of  its  cilia,  turns  to- 
ward the  aboral  side  (that  opposite  to  the  oral  groove) 
by  increasing  the  beat  of  the  oral  cilia  and  lessening  the 
compensating  rotation,  and  continues  on  a  forward  course 
that  is  now  at  an  angle  with  its  former  line  of  motion. 
If  this  new  course  carries  it  clear  of  the  stimulus,  it  con- 
tinues on  its  way ;  if  not,  repeated  contact  with  the  stim- 
ulus causes  a  second  reaction,  the  Paramecium  always 
turning  in  the  same  direction,  so  that  ultimately  it  avoids 
the  source  of  stimulation  (361,  378)  (Fig.  4).  Differing 
strengths  of  stimulus  produce  the  reaction  with  different 
degrees  of  violence.  When  a  very  strong  stimulus  is  en- 
countered, the  animals  "  respond  first  by  swimming  a  long 
way  backward,  thus  removing  themselves  as  far  as  possible 
from  the  source  of  stimulation.  Then  they  turn  directly 
toward  the  aboral  side,  —  the  rotation  on  the  long  axis 
completely  ceasing.  In  this  way  the  animal  may  turn 
directly  away  from  the  drop  [the  stimulus]  and  retrace  its 
course"  (378,  p.  50).  On  the  other  hand,  when  the  stim- 
ulus is  very  weak  the  reaction  may  be  reduced  to  the 


Sensory  Discrimination:   The  Chemical  Sense       65 

following  form:  the  Paramecium  "  merely  stops,  or  pro- 
gresses more  slowly,  and  begins  to  swing  its  anterior  end 
about  in  a  circle."  As  long  as  it  does  not  thus  get  out  of 
range  of  the  stimulus,  the  movement  is  continued.  "When 
the  anterior  end  is  finally  pointed  in  a  direction  from  which 
no  more  of  the  stimulating  agent  comes,  the  Paramecium 
swims  forward"  (378,  p.  51).  Evidently,  however,  these 
are  but  differing  degrees  of  a  reaction  whose  essential 
features  are  the  same. 
While  Paramecium  definitely  avoids  by  means  of  this 


/.  JL 

FIG.  4.  —  Negative  reaction  of  Paramecium.     A  is  the  source  of  stimulation. 
1-6  are  the  successive  positions  of  the  animal.    After  Jennings  (378). 

negative  reaction  certain  chemicals  introduced  into  the 
water,  it  shows  a  tendency  to  collect  in  the  neighborhood 
of  others.  Such  is  the  case  with  weak  acids,  with  a  bubble 
of  oxygen  if  air  has  been  long  excluded  from  the  slide,  and 
with  carbon  dioxide,  which  in  water  of  course  produces 
acid  (378).  Jennings  pointed  out  that  the  inclination  of 
Paramecium  to  gather  in  groups  is  very  likely  due  to  the 
attraction  for  them  of  the  carbon  dioxide  which  they  ex- 
crete. But  he  has  also  shown  that  this  "attraction"  to 
certain  chemicals  does  not  mean  the  presence  of  a  special 


66 


The  Animal  Mind 


positive  reaction.  The  fact  is  that  when  the  animals 
collect  in  a  drop  of  weak  acid,  for  example,  they  are  not 
drawn  toward  the  acid.  They  simply  happen,  in  their 
ordinary  movements,  to  swim  into  it,  and  on  entering  it 
show  no  disturbance  whatever.  But  when  they  come  to 
the  edge  of  the  drop  on  their  way  out,  they  give  the  nega- 
tive reaction  to  the  surrounding  water.  In 
this  way  they  are,  as  it  were,  trapped  within 
the  drop. 

The  nearest  analogue  to  a  positive  reaction 
in  Paramecium  consists  in  the  fact  that  some- 
times, when  they  come  into  contact  with  a 
solid,  instead  of  darting  backward,  the  animals 
merely  cease  moving,  and  extending  stiffly  the 
cilia  which  touch  the  object,  remain  at  rest 
(Fig.  5).  The  utility  of  this  behavior  is  that 


Jennings 
(378). 


FIG.  5.— 
Positive 

thigmotaxis  aroun(i  decaying  vegetable  matter,  the  kind  of 
dum.  After  solid  oftenest  found  in  the  animal's  ordinary 
environment,  there  is  apt  to  be  a  supply  of 
food  in  the  way  of  bacteria;  it  is  a  good 
anchorage.  What  characteristics  of  the  stimulus  determine 
that  this  "contact  reaction,"  rather  than  the  negative  re- 
action, shall  be  given  ?  Does  weak  mechanical  stimulation 
occasion  it,  as  happens  with  Amoeba's  positive  reaction? 
Evidence  in  favor  of  this  is  offered  by  the  fact  that  the  con- 
tact reaction  is  more  likely  to  occur  if  the  animal  comes 
against  the  solid  when  swimming  rather  slowly.  Jennings  re- 
ports also  that  individuals  vary.  "Often  all  the  individuals 
in  a  culture  are  thus  inclined  to  come  to  rest,  while  in  an- 
other culture  all  remain  free-swimming,  and  give  the  avoid- 
ing reaction  whenever  they  come  in  contact  with  a  solid" 
(378,  p.  60).  This  would  suggest  that  some  individuals 
are  in  a  state  of  greater  excitability  than  others,  so  that  a 


Sensory  Discrimination:   The  Chemical  Sense    67 

given  stimulus  acts  more  strongly  upon  them.  On  the 
other  hand,  there  is  a  possibility  that  qualitative  as  well 
as  intensive  differences  in  the  stimulus  are  responsible  for 
the  contrasting  reactions.  "In  general,"  says  Jennings, 
Paramecium  "shows  a  tendency  to  come  to  rest  against 
loose  or  fibrous  material ;  in  other  words,  it  reacts  thus  to 
material  with  which  it  can  come  in  contact  at  two  or 
more  parts  of  the  body  at  once.  To  smooth,  hard  materials, 
such  as  glass,  it  is  much  less  likely  to  react  in  this  manner" 
(378,  p.  61).  Perhaps,  then,  the  spatial  distribution  of  the 
stimulus  over  several  points  of  the  body  surface  increases 
the  probability  of  a  contact  rather  than  an  avoiding  re- 
action. 

What,  now,  of  the  food-taking  reaction  in  Paramecium : 
does  it  show  evidence  of  the  existence  of  chemical  dis- 
crimination? When  the  animal  finds  itself  in  surround- 
ings where  certain  presumably  injurious  chemicals,  es- 
pecially alkalis,  are  present,  it  gives  its  typical  negative 
reaction.  If  this  should  be  called  evidence  of  a  special 
chemical  sense,  we  should  be  forgetting  our  general  prin- 
ciple that  only  unlike  reactions  constitute  behavior  indi- 
cating sensory  discrimination.  Since  Paramecium  reacts 
in  the  same  way  to  strong  mechanical  stimulation  and  to 
certain  chemical  stimulations,  there  is  no  reason  for  assum- 
ing a  discrimination  between  chemical  and  mechanical 
stimuli.  If  it  can  be  shown  that  the  reaction  is  a  localized 
one,  that  the  cilia  which  surround  the  mouth  reverse  the 
direction  of  their  beat  when  certain  kinds  of  particles 
strike  upon  them,  with  the  result  that  these  particles  are 
thrown  out,  then  the  question  as  to  the  existence  of  a 
chemical  discrimination  would  depend  on  whether  the 
rejected  particles  are  chemically  unlike  those  which  are 
accepted,  or  different  only  in  size  or  mechanical  consist- 


68  The  Animal  Mind 

ency.  Jennings  (378)  reports  no  such  rejection  of  unsuit- 
able particles  in  the  case  of  Paramecium,  but  Metalnikow 
(485,  486)  says  that  when  Paramecia  have  been  kept  for 
some  time  in  water  containing  carmine  grains  they  cease 
to  swallow  them ;  the  evidence  being  that  fewer  and  fewer 
grains  are  found  in  the  animals.  Schaeffer  (656)  thinks 
this  result  is  due  to  the  mechanical  change  in  carmine 
grains  that  have  been  long  in  the  water,  which  become 
stuck  together  in  the  mucus  excreted  by  the  Paramecia. 
Metalnikow  (487)  however  finds  that  when  fresh  carmine 
is  used  the  Paramecia  avoid  it  apparently  as  a  result  of 
their  previous  surfeit,  and  that  when  particles  of  aluminum 
are  used  instead  of  carmine  they  acquire  a  discrimination 
against  these  even  more  quickly.  He  therefore  feels  con- 
vinced that  the  discrimination  is  a  chemical  one. 

Stentor  is  a  ciliate  protozoon  which  spends  a  part  of  its 
existence  anchored  by  a  long  extension  of  its  body,  like 
the  stem  of  a  flower :  at  times  it  pulls  this  up  and  swims 
off.  Food  is  taken  in  by  the  whirl  of  cilia  around  the 
mouth,  and  may  be  rejected  by  a  reversal  of  the  direction 
of  this  whirl.  Schaeffer  (656)  says  that  Stentor  discrim- 
inates not  only  between  organisms  and  inedible  particles, 
but  between  different  kinds  of  organisms;  he  thinks, 
however,  that  the  basis  of  discrimination  is  not  chemical, 
because  food  soaked  in  a  variety  of  chemicals  is  readily 
taken,  while  jelly  made  of  food  organisms  is  rejected.  He 
believes  the  discrimination  rests  probably  on  several  me- 
chanical factors  in  combination,  for  example,  size,  weight, 
form,  and  surface  texture,  no  one  of  which  is  alone  suf- 
ficient to  determine  the  choice.  On  the  other  hand  Lund 
(446),  observing  another  ciliate  named  Bursaria,  finds  that 
this  organism  will  reject  yolk  of  egg  particles  if  they  have 
been  treated  with  certain  dyes,  and  concludes  that  the  basis 


Sensory  Discrimination:   The  Chemical  Sense     69 

of  discrimination  is  chemical.  Lacrymaria,  another  ciliate, 
tests  with  its  'head'  "every  object  within  reach  and  rejects 
all  those  which  cannot  serve  as  food.  It  does  not  swallow 
inorganic  substances,  carmine,  or  ink  particles  and  the 
like.  This  protozoon  unquestionably  exercises  selection  in 
feeding"  (469,  p.  243),  but  the  basis  of  the  selection  is  not 
determined.  Didinium  is  a  ciliate  which  has  a  peculiarly 
modified  seizing  organ,  but  the  only  selection  of  food  which 
it  makes  rests  on  the  fact  that  this  organ  will  adhere 
to  the  surface  of  some  organisms  and  not  to  that  of  others 
(466).  Two  other  protozoa,  Actinobolus  radians  and 
Spathidium  spathula,  have  each  so  far  refined  the  process 
of  selection  of  food  that  they  swallow  only  one  kind  of 
organism.  Actinobolus,  an  anchored  form,  awaits  its 
destined  prey,  and  Spathidium  selects  it  in  freely  swimming 
about ;  but  as  to  whether  the  prey  is  recognized  by  chemical 
or  by  mechanical  features  we  have  no  information  (499). 

§  1 6.   The  Chemical  Sense  in  Ccelenterates 

The  lowest  of  the  Metazoa,  or  many-celled  animals, 
are  the  ccelenterates.  Although  externally  the  forms  of 
different  families  of  ccelenterates  differ  widely,  yet  the 
general  plan  of  structure  is  the  same  in  all :  the  body  of 
the  typical  ccelenterate  is  a  hollow  sac,  whose  walls  con- 
sist of  two  layers  of  cells,  food  being  taken  into  a  mouth 
at  one  end  of  the  sac,  and  the  arrangement  of  cells  being 
on  the  plan  of  circular  symmetry.  In  the  phylum  of  the 
ccelenterates  are  included  sea-anemones,  jellyfish,  the  little 
green  or  yellow  Hydra,  sponges,  corals,  and  ctenophores. 

Hydra  (Fig.  6),  one  of  the  simplest  ccelenterates,  shows 
a  food  reaction  distinct  from  the  contact  reaction.  Me- 
chanical stimulation  is  followed  by  withdrawal  of  the  ten- 


The  Animal  Mind 


tacles  and  by  contraction  of  the  stem.  This  behavior 
may  be  called  a  negative  or  avoiding  reaction,  and  no 
positive  reaction  to  a  mechanical  stimulus  has  been  ob- 
served. The  food- taking  reaction,  on  the  other  hand, 
consists  in  the  seizing  of  the  food  by  the  tentacles.  It 

seems  to  be  given  in  re- 
sponse to  a  combination 
of  chemical  with  me- 
chanical stimulation, 
such  as  is  offered  by  con- 
tact with  a  solid  edible 
object  (751  a).  Shall 
we  say  that  Hydra  pos- 
sesses, then,  a  food 
sensation  and  a  con- 
tact sensation  that  are 
distinguishable  in  its 
consciousness,  provided 
such  consciousness  ex- 
ists? It  may  be  that 
the  contrast  between 
the  two  is  more  nearly 

FIG.  6.— Hydra,    mth,  mouth;  /,  tentacle,      analogous    to     that     be- 
After  Parker. 

tween  pleasantness  and 

unpleasantness  in  our  own  experience,  for  the  food- 
taking  reaction  in  Hydra  is  the  only  form  of  the  positive 
reaction,  and  the  response  to  mere  contact  is  distinctly 
negative  in  character.  The  influence  of  physiological 
condition  in  Hydra's  reactions  is  shown  by  the  fact  that 
although  ordinarily  the  food  response  is  brought  about 
only  by  contact  with  food,  if  the  animal  is  very  hun- 
gry any  chemical  stimulation,  even  quinine,  will  produce 
it  (751  a).  This  blunting  of  discrimination  has,  of  course, 


Sensory  Discrimination:   The  Chemical  Sense      71 

the  adaptive  aspect  that  the  starved  animal  can  afford  to 
lose  no  chances,  and  suggests  the  analogy  from  our  own 
experience  of  the  loss  of  intellectual  discrimination  in  mo- 
ments of  intense  emotion.  For  the  emotion  too  repre- 
sents a  situation  where  the  organism  cannot  afford  to  lose 
chances  by  hesitating  in  reaction  long  enough  for  nice 
discrimination. 

In  Tubularia  crocea,  a  ccelenterate  belonging  to  the  family 
of  hydroids  which  form  colonies  of  many  individuals  on  a 
common  stem,  food  and  contact  stimuli  do  not  produce 
different  reactions,  but  have  different  degrees  of  efficiency 
in  bringing  about  response.  When  a  grain  of  sand  was 
placed  in  contact  with  the  tentacles  on  one  side  and  a  bit 
of  meat  in  a  corresponding  position  on  the  other  side,  the 
reaction  was  almost  invariably  in  the  direction  of  the 
meat.  Filtered  meat  juice  allowed  to  flow  upon  the  distal 
tentacles  produced  a  reaction  82  per  cent,  of  the  time,  while 
carmine  water  was  effective  only  15  per  cent,  of  the  time. 
Further,  if  the  distal  tentacles  were  touched  several  times 
with  a  needle,  they  remained  closed;  but  if  the  second 
stimulus  used  was  a  piece  of  meat,  the  tentacles  opened  out 
and  waved  about  (564).  Whether  in  such  a  case  as  this 
the  possible  conscious  accompaniments  of  the  responses  are 
to  be  regarded  as  qualitatively  different  sensations,  or  only 
as  different  degrees  of  intensity  of  the  same  sensation,  it 
is  difficult  to  say.  Another  hydroid,  Corymorpha  palma, 
gives  no  response  whatever  to  meat  juice ;  only  irritating 
chemicals  produce  reactions,  whose  character  appears  to 
be  tactile  (714). 

In  the  sea-anemones  or  actinians  we  find  behavior  in 
response  to  food  stimulation  as  distinguished  from  contact 
stimulation  varying  in  different  representatives  of  the 
group.  Generally  speaking,  the  food  reaction  seems  to 


72  The  Animal  Mind 

be  more  marked  than  the  contact  reaction.  W.  H.  Pollock 
a  number  of  years  ago  reported  his  observation  that  cer- 
tain unnamed  sea-anemones  opened  out  if  food  were  sus- 
pended near  them  in  the  water,  and  referred  the  phenom- 
enon to  aa  sense  of  smell"  (609).  Adamsia  rondeleti  winds 
its  tentacles  around  bits  of  sardine  meat  and  passes  them 
from  tentacle  to  tentacle  toward  the  mouth.  When  balls 
of  filter  paper  softened  with  sea  water  are  substituted,  the 
feeding  reaction  is  wholly  lacking.  Either  the  tentacles 
fail  to  react  at  all,  or  the  ball  is  "felt  of"  slowly  with  no 
attempt  to  seize  it,  or  it  is  momentarily  seized  and  then 
dropped.  If  the  paper  ball  be  soaked  in  fish  juice,  on  the 
other  hand,  it  is  seized  as  eagerly  as  the  fish  meat.  A 
negative  reaction,  consisting  in  the  withdrawal  of  the 
tentacles  affected,  may  be  produced  by  applying  a  bit  of 
paper  soaked  in  quinine  solution  or  by  the  discharge  of 
quinine  solution  from  a  pipette  near  the  tentacles  (427, 
518).  A  peculiar  form  of  negative  reaction  has  been  ob- 
served in  Adamsia,  and  more  strikingly  in  Cerianthus,  when 
a  paper  ball  soaked  in  fish  juice  has  been  passed  from  ten- 
tacle to  tentacle  till  it  has  nearly  reached  the  mouth.  The 
process  is  suddenly  reversed,  and  the  ball  is  passed  back 
from  one  tentacle  to  another  till  it  reaches  the  outside  edge 
and  is  dropped  off.  Nagel,  the  observer,  thinks  the  stim- 
ulus for  this  change  of  reaction  is  the  gradual  wearing  off 
of  the  "sapid  parts"  of  the  ball  during  its  passage  toward 
the  mouth  —  it  might  be  the  squeezing  out  of  the  meat 
juice  —  and  calls  special  attention  to  the  fact  that  the 
reaction  whereby  the  paper  is  got  rid  of  is  wholly  different 
from  the  ordinary  reaction  of  a  tentacle  to  mechanical 
stimulation,  which,  as  we  have  seen,  does  not  involve  seiz- 
ing the  object  at  all.  A  tentacle  touched  by  a  bit  of  moist- 
ened filter  paper  ordinarily  responds,  if  at  all,  by  a  mere 


Sensory  Discrimination:   The  Chemical  Sense     73 

contraction  without  the  winding  seizure  of  the  object. 
Touched  by  the  same  object  "  handed  on"  to  it  by  a  tentacle 
nearer  the  mouth  than  itself,  it  seizes  the  paper  and  passes 
it  on  to  the  tentacle  beyond  it.  The  cause  of  this  differ- 
ence in  behavior  seems  to  lie  in  the 
processes  that  have  been  taking 
place  just  previously.  Nagel  does 
not  hesitate  to  say  that  a  psychic 
process  must  be  involved,  but  its 
details  are  not  easy  to  construct 


Another    sea-anemone,   Aiptasia, 
has  but  one  ring  of  tentacles,  and    FIG.  7.- Metridium.   After 

Parker. 

like    Tubularia    crocea,    instead    of 

showing  different  responses  to  contact  stimulation  alone 
and  to  contact  plus  food  stimulation,  it  merely  reacts  with 
greater  emphasis  to  the  latter.  In  both  cases  the  tentacles 
wind  around  the  object,  contract,  and  direct  themselves 
toward  the  mouth  (521).  Again  the  question  arises  whether 
the  possible  accompanying  sensations  differ  in  quality  or 
only  in  intensity.  One  species  of  Aiptasia,  A.  annulata, 
however,  does  react  differently  to  filter  paper  soaked  in  crab 
juice  and  to  plain  filter  paper  (374),  showing  that  even 
within  a  genus  the  capacity  for  stimulus  discrimination 
may  differ.  In  like  manner  one  sea-anemone,  Actinia, 
will  take  filter  paper  soaked  in  acetic  acid,  while  another, 
Tealia,  rejects  it  (228). 

Metridium,  a  common  sea-anemone  of  our  coasts,  has 
its  tentacles  covered  with  cilia  which  have  a  continual 
waving  motion  toward  the  tip  of  the  tentacle  (Fig.  7). 
If  particles  of  an  inedible  substance  are  dropped  on  a  ten- 
tacle, no  definite  reaction  occurs,  but  the  particles  are 
carried  by  the  ordinary  motion  of  the  cilia  out  to  the  ten- 


74  The  Animal  Mind 

tacle  tip,  where  they  drop  off.  When  a  bit  of  crab  meat, 
or  some  meat  juice,  is  dropped  on  a  tentacle,  the  latter 
contracts  and  curls  over  with  the  tip  directed  toward  the 
mouth.  The  ciliary  movement  continuing  in  its  usual  di- 
rection now  of  course  carries  the  food  toward  the  mouth. 
Applying  food  to  the  lips  on  either  side  of  the  mouth  causes 
a  different  response.  The  cilia  on  these  lips  ordinarily 
wave  outwards;  when  food  is  brought  in  contact  with 
them  their  motion  is  reversed,  and  the  food  is  thus  passed 
into  the  mouth.  In  Metridium,  then,  there  is  no  specific 
rejecting  reaction  for  inedible  substances  (533). 

Various  instances  of  the  effect  of  physiological  condition 
upon  response  to  food  stimulation  in  sea-anemones  have 
been  noted.  Adamsia  loses  the  power  to  discriminate 
between  edible  and  inedible  substances  when  very  hun- 
gry (521).  Sagartia  davisi  will  also  swallow  inedible  sub- 
stances if  hungry  enough  (715).  Stoiachactis  helianthus 
will  give  either  a  positive  or  a  negative  reaction  to  food 
according  to  its  condition  of  hunger  or  satiety  (374).  The 
reaction  of  Metridium  to  food  may  vary  decidedly  with  the 
degree  of  hunger  (3),  although  it  will  continue  taking  food 
as  long  as  the  process  is  mechanically  possible  (378).  Fa- 
tigue has  also  been  shown  to  affect  the  food  responses  of 
Metridium  and  other  sea-anemones;  specimens  that  have 
been  fed  meat  and  filter  paper  alternately  will  after  a  time 
refuse  to  take  filter  paper  (374,  521,  533).  This  behavior 
was  thought  by  Nagel  to  indicate  that  the  animal  had  dis- 
covered the  deception  practised  upon  it ;  but  according  to 
Gee  (256)  the  real  cause  is  increased  secretion  of  mucus, 
which  lowers  the  responsiveness  of  the  animal.  This 
effect  would  naturally  be  felt  first  in  response  to  weak 
stimuli. 

As  regards  the  localization  of  the  sensitive  elements, 


Sensory  Discrimination:   The  Chemical  Sense     75 

authorities  disagree,  and  probably  species  differ.  Nagel 
finds  the  tentacles  most  sensitive  (521) ;  Loeb  observed 
that  the  stump  of  the  animal  has  discriminative  reactions 
(427),  while  Fleure  and  Walton  state  that  in  the  species 
tested  by  them  the  mouth-region  is  most  responsive  to 
chemical  stimulation  (228). 

A  certain  amount  of  discrimination  between  mechanical 
stimuli  is  ascribed  to  these  animals  by  Romanes.  "I 
have  observed,"  he  says,  "that  if  a  sea-anemone  is  placed 
in  an  aquarium  tank  and  allowed  to  fasten  upon  one  side 
of  the  tank  near  the  surface  of  the  water,  and  if  a  jet  of 
sea  water  is  made  to  play  continuously  and  forcibly  upon 
the  anemone  from  above,  the  result  of  course  is  that  the 
animal  becomes  surrounded  with  a  turmoil  of  water  and 
air  bubbles.  Yet  after  a  short  time  it  becomes  so  accus- 
tomed to  this  turmoil  that  it  will  expand  its  tentacles  in 
search  of  food,  just  as  it  does  when  placed  in  calm  water. 
If  now  one  of  the  expanded  tentacles  is  gently  touched 
with  a  solid  body,  all  the  others  close  around  that  body  in 
just  the  same  way  as  they  would  were  they  expanded  in 
calm  water"  (642,  p.  48),  although  the  solid  stimulus  is 
decidedly  less  intense  than  that  offered  by  the  bubbles. 
Similarly,  Fleure  and  Walton  find  that  certain  species 
show  little  reaction  to  accidental  contact  with  a  pebble 
that  is  moved,  but  react  quickly  to  a  finger  (228). 

The  body  of  a  typical  medusa  or  jellyfish  consists  of  a 
bell-shaped  "umbrella"  from  the  edge  of  which  tentacles 
depend.  Hanging  from  the  middle  like  the  clapper  of  the 
bell  or  the  handle  of  the  umbrella  is  the  manubrium,  at 
the  end  of  which  is  the  mouth.  In  the  medusa  Carmarina 
hastata  no  differentiation  in  reaction  to  contact  and  food 
stimulation  appears,  merely  a  readier  response  of  the  ten- 
tacles to  the  latter ;  but  we  do  find  whatever  evidence  for 


76  The  Animal  Mind 

the  existence  of  a  specific  sensation  quality  is  furnished  by 
localized  sensitiveness,  for  the  skin  of  the  under  side  of 
the  umbrella,  and  of  the  manubrium,  is  very  sensitive  to 
mechanical  stimulation,  and  wholly  insensitive  to  chemical 
stimulation,  while  the  tentacles,  as  has  just  been  stated, 
react,  by  shortening  and  twisting  themselves  about  the 

object,  more  readily  to 
chemical  than  to  me- 
chanical stimulation.  A 
mechanical  stimulus  ap- 
plied to  any  part  of  the 
under  edge  of  the  um- 
brella produces  after 
from  one  to  three 
seconds  a  movement 

of  the  manubrium  tip  toward  the  point  stimulated  (519, 
521). 

The  little  medusa  Gonionemus  murbachii  (Fig.  8)  shows, 
on  the  other  hand,  two  well-defined  different  responses  to 
special  stimulation :  motor  reactions  and  food-taking 
reactions.  The  motor  or  swimming  reactions  are  given 
in  response  to  mechanical  stimulation  and  to  the  presence 
of  food  near  the  animal  in  the  water ;  but  the  food-taking 
reaction  occurs  only  in  response  to  food  (solution  of  fish 
meat) ;  very  rarely  a  weak  inorganic  chemical  stimulus 
will  produce  the  beginning  of  the  response.  An  impor- 
tant exception  to  the  usual  inefficacy  of  mechanical  stimuli 
in  bringing  about  the  feeding  reaction  occurs  when  a  moving 
mechanical  stimulus  is  used;  this  very  quickly  produces 
the  early  stages  of  the  food-taking  response.  Special 
reactions  to  stimuli  in  motion  are  widespread  throughout 
the  animal  kingdom ;  their  significance  will  be  discussed 
in  the  chapter  on  Space  Perception.  The  food- taking  re- 


Sensory  Discrimination:   The  Chemical  Sense     77 

sponse  in  Gonionemus  shows  a  marked  coordination  of 
movements;  if  the  food  touches  one  or  more  tentacles, 
these  contract  and  twist  about  it ;  they  then  bend  toward 
the  manubrium,  and  the  margin  of  the  bell  also  bends 
in;  the  manubrium  swings  over  toward  the  bell  and  en- 
velops the  food  with  its  lips  (802). 

Another  coelenterate  whose  reactions  to  chemical  stimu- 
lations have  been  observed  is  the  ctenophore  Beroe  ovata. 
Its  body  is  an  elongated  oval,  with  longitudinal  ciliated 
ridges,  having  the  mouth  slit  at  the  end  which  is  normally 
uppermost  when  the  animal  is  at  the  surface  of  the  water, 
and  at  the  opposite  end  an  otolith  or  statolith  organ  lying 
between  two  flattened  " polar  plates."  The  significance  of 
this  organ  will  be  considered  later.  The  aboral  region  is 
far  more  sensitive  than  any  other  to  mechanical  stimu- 
lation ;  the  slightest  touch  on  one  of  the  polar  plates  causes 
the  animal  to  shorten  itself  and  fold  in  the  plates.  The 
aboral  end,  being  the  hind  end  of  the  creature,  is  not 
usually  brought  into  contact  with  objects.  Nagel,  who 
studied  the  animal,  suggests  that  this  region,  being  sensi- 
tive to  changes  in  pressure,  may  enable  the  animal  to  right 
itself  when  it  rises  to  the  surface  with  the  aboral  end  up, 
as  the  change  from  water  to  air  pressure  could  not  fail  to 
stimulate  the  polar  plates.  Nagel  apparently  made  no 
experiments  on  the  behavior  of  Beroe  with  reference  to 
food  stimuli ;  for  chemical  stimulation  he  used  picric  acid, 
dilute  hydrochloric  acid,  quinin,  strychnin,  saccharin, 
coumarin,  vanillin,  and  naphthalin.  To  all  these  un- 
wonted stimuli  the  animal  responded  by  some  form  of 
negative  reaction,  indicating  possible  unpleasant  feeling. 
The  edges  of  the  mouth,  where  the  nerves  end  in  bulb-like 
structures,  reacted  to  quinin,  vanillin,  and  coumarin  by 
stretching  the  mouth  into  a  circular  form  instead  of  its 


7 8  The  Animal  Mind 

usual  slit-like  shape,  suggesting  an  effort  to  get  rid  of  the 
stimulus.  Precisely  similar  reactions  were  produced  by 
stimulation  with  lukewarm  water.  Nagel  concludes  that 
the  organs  for  chemical  and  thermal  stimulation  are  iden- 
tical; whether  the  sensation  qualities  are  different  is,  he 
thinks,  an  open  question.  There  is  at  least  no  evidence 
that  they  are  different  (519,  521). 

§  17.   The  Chemical  Sense  in  Flatworms 

Next  to  the  ccelenterates  zoologists  place  the  phylum  of 
the  Platyhelminthes  or  flatworms,  which  possess  a  bilaterally 
instead  of  a  radially  symmetrical  structure.  Many  repre- 
sentatives of  the  group  are  parasitic,  and  so  far  as  the  writer 
is  aware,  no  extended  study  of  the  reactions  of  these  forms 
to  stimulation  has  been  made.  Most  of  our  knowledge  in 
regard  to  the  sensory  life  of  the  flatworms  in  confined  to 
the  class  Turbellaria,  including  the  common  freshwater 
and  marine  planarians.  These  are  small  slow-moving 
creatures  which  crawl  about  on  solid  objects  under  water 
or  on  films  covering  the  surface.  The  mouth  is  situated 
on  the  ventral  side  of  the  body,  sometimes  quite  far  re- 
moved from  the  head  end  (Fig.  9).  One  chief  interest  of 
planarians  to  physiologists  has  lain  in  their  remarkable 
power  to  regenerate  parts  lost  by  mutilation. 

Planaria  maculata,  a  common  freshwater  planarian,  re- 
sponds to  stimulation  by  two  forms  of  negative  reaction, 
a  positive  reaction,  and  a  feeding  reaction.  The  negative 
and  positive  responses  are  given  either  to  mechanical  or 
to  chemical  stimuli,  the  former  being  produced  by  strong, 
the  latter  by  weak  stimulation.  Hence  they  do  not  sug- 
gest correlation  with  qualitatively  different  sensation  con- 
tents, but  rather  with  unpleasantness  and  pleasantness. 


Sensory  Discrimination:   The  Chemical  Sense     79 


The  two  forms  of  negative  reaction  correspond  to  differ- 
ences in  the  location  of  the  stimulus.  If  the  head  end  of 
the  body  is  stimulated  strongly  on  one  side,  the  head  is 
turned  away  from  that  side.  If  the  posterior  part  of  the 
body  is  strongly  stimulated,  the  animal 
makes  powerful  forward  crawling  move- 
ments. The  significance  of  local  differences 
in  stimulation  for  response  and  for  possible 
consciousness,  again,  will  more  properly  be 
discussed  in  a  later  chapter.  As  has  just 
been  said,  both  weak  chemical  and  weak 
mechanical  stimulation  cause  Planaria  macu- 
lata  to  give  a  positive  reaction  by  turning 
its  head  in  the  direction  of  the  stimulus, 
which  need  not  be  in  actual  contact  with 
the  body  (561).  A  planarian  will  follow 
an  object  such  as  the  point  of  a  pin  moved 
in  front  of  it,  and  one  planarian  will  follow 
the  trail  of  another  that  happens  to  come 
within  the  proper  distance.  Similarly,  the 
neighborhood  of  food  will  cause  the  animal 
to  turn  toward  it.  Bardeen  has  suggested 
that  the  so-called  "auricular  appendages," 
two  small  movable  prominences  on  the 
animal's  back  near  the  head  end,  which  are 
specially  sensitive  to  touch,  may  be  "deli- 
cate organs  capable  of  stimulation  by  slight 
currents  in  the  water  set  up  by  the  minute  organisms 
that  prey"  upon  the  animal's  food;  so  that  the  pos- 
itive reaction  when  given  to  food  may  be  really  a  re- 
sponse to  mechanical  stimulation  (20).  As  Pearl,  however, 
found  that  chemicals,  diffused  in  the  water,  would  produce 
positive  responses  (561),  it  is  probable  that  Planaria 


FIG.  9.  —  Pla- 
narian, dorsal 
view.  After 
Woodworth. 


8o  The  Animal  Mind 

maculata  is  directly  sensitive  to  chemical  stimulation, 
though  it  responds  thereto  in  the  same  way  as  to  mechanical 
stimulation.  A  land  planarian,  Geodesimus  bilineatus,  is 
reported  by  Lehnert  to  perceive  food  at  distances  from 
four  to  five  times  the  length  of  its  body,  and  he  does  not 
describe  the  positive  reaction  as  given  in  response  to  any 
other  than  food  stimulation  (417). 

The  food-taking  reaction  in  Planaria  maculata  is  made 
under  the  influence  of  combined  mechanical  and  chemical 
stimuli,  in  contact  with  the  pharynx  or  the  ventral  side  of 
the  animal.  When  an  object  which  has  occasioned  the 
positive  reaction  is  reached,  the  head  folds  over  it  and 
grips  it,  contracting  so  as  to  squeeze  it.  The  substance 
being  thus  brought  into  contact  with  the  pharynx,  swal- 
lowing movements  are  produced  if  the  proper  stimulus  is 
given.  In  Microstoma  caudatum  the  organ  of  the  chemical 
sense  has  been  held  to  be  sensory  epithelium  in  the  floor 
of  the  pharynx  (398).  Bardeen  was  inclined  to  think 
that  contact  with  a  soft  substance  constituted  the  proper 
stimulus,  as  he  found  that  hard  particles  placed  on  the 
pharynx  were  not  swallowed  (20).  Pearl,  however,  be- 
lieves that  mechanical  and  chemical  stimulation  must  com- 
bine. The  former  alone  does  not  suffice,  for  swallowing 
movements  are  not  evoked  when  one  planarian  crawls 
over  another;  the  latter  alone  is  insufficient,  for  placing 
the  animal  in  a  sugar  solution  has  no  effect.  If  chemical 
and  mechanical  stimulation  are  united,  the  reaction  is 
given  whether  the  chemical  is  edible  or  not;  Pearl  found 
it  occurring  in  response  to  sodium  carbonate  (561). 

Evidence  of  the  influence  of  physiological  condition  upon 
the  reactions  of  planarians  is  furnished  by  the  fact  that  the 
resting  planarian  shows  a  decidedly  lowered  susceptibility 
to  stimulation.  Bardeen  found  that  if  the  animal  was  not 


Sensory  Discrimination:   The  Chemical  Sense     81 

already  in  motion,  it  gave  no  positive  response  to  food  in 
its  neighborhood  (20). 


§  1 8.   The  Chemical  Sense  in  Annelids 

In  our  own  experience,  as  has  been  said,  the  "food  sense" 
is  represented  by  the  two  senses,  taste  and  smell,  the  stimu- 
lus for  the  one  being  fluid,  and  that  for  the  other  gaseous, 
so  that  the  latter  enables  us  to  perceive  objects  at  a  dis- 
tance. For  water-dwelling  animals,  such  as  most  of  those 
whose  behavior  we  have  been  describing,  the  distinction 
evidently  cannot  well  be  drawn.  If  such  an  animal  per- 
ceives food  at  a  distance,  the  stimulus  is  necessarily  dif- 
fused through  the  water,  and  Lloyd  Morgan  has  proposed 
the  term  " telaesthetic  taste"  for  the  sense  which  makes 
such  perception  possible  (504,  p.  256).  The  term  indi- 
cates that  this  sense  corresponds  to  taste  in  an  air-dwelling 
animal  because  the  stimulus  is  fluid,  but  differs  in  that  it 
allows  perception  of  a  distant  object,  as  taste  -in  the  or- 
dinary sense  does  not.  In  the  most  familiar  representative 
of  the  Annelida  or  segmented  worms,  the  common  earth- 
worm, as  in  the  land  planarian,  a  distinction  analogous  to 
that  between  smell  and  taste  in  our  own  sensory  experience 
may  be  made ;  in  the  leeches  and  marine  annelids  it  cannot. 

Gentle  and  continuous  mechanical  stimulation  produces 
in  the  earthworm  "  positive  thigmotaxis  " ;  that  is,  the  ani- 
mals have  a  tendency  to  crawl  and  lie  along  the  surface  of 
solids  (686).  That  there  is  some  discrimination  of  edible 
from  inedible  substances  when  in  contact  with  the  body 
Darwin  thought  probable  from  the  apparent  preference 
of  the  worm  for  certain  kinds  of  food  (171).  In  the  earth- 
worm Allolobophorafcetida  we  find  a  differentiated  response 
to  contact  and  chemical  stimulation.  These  worms  live 


82  The  Animal  Mind 

in  barnyard  manure.  When  placed  on  scraps  of  shredded 
filter  paper  moistened  with  water  they  refuse  to  burrow; 
when  the  filter  paper  is  wet  with  a  decoction  of  the  manure 
they  burrow  as  soon  as  they  come  into  contact  with  it. 
The  adequate  stimulus  for  burrowing  is  thus  a  combined 
mechanical  and  chemical  one;  the  chemical  stimulus 
alone  is  insufficient,  for  filter  paper  thus  prepared  has  no 
effect  on  the  worms  unless  they  are  actually  in  contact 
with  it  (686).  Using  the  human  terms,  the  case  is  one 
of  taste  rather  than  smell.  Nagel  suggests  that  the  earth- 
worm's chief  use  for  a  chemical  sense  is  to  help  it  find  the 
moisture  which  is  necessary  to  its  life  (522) ;  but  curiously 
enough  Allolobophora  fcetida  seems  to  have  no  power  of 
doing  this  from  a  distance.  Smith  found  that  a  worm 
would  crawl  around  a  wet  spot  on  paper  until  its  skin  dried, 
without  crawling  into  it.  If  by  accident  it  happened  to 
touch  the  moist  place,  it  would  enter  and  remain  there 
(686).  Parker  and  Parshley  (555)  find  that  the  head 
end  of  the  worm  is  negatively  stimulated  by  contact  with 
a  dry  surface,  and  will  withdraw  soon  after  such  contact. 
There  seems  no  satisfactory  evidence  that  worms  respond 
to  chemical  stimulation  from  a  distance  by  positive  re- 
actions, although  Darwin  believed  that  they  found  buried 
food  by  "the  sense  of  smell"  (171).  Chemical  stimuli 
not  in  contact  with  the  body  do  produce  negative  reactions 
(522),  but  these  reactions  do  not  differ  from  the  responses 
to  strong  mechanical  stimulation.  They  are  of  various 
forms  —  turning  aside,  withdrawing  into  the  burrow  if 
the  tail  is  already  inserted,  squirming,  and  so  on,  the  dif- 
ferences being  correlated  with  differences  in  the  intensity 
and  location  of  the  stimulus  and  in  the  excitability  (physi- 
ological condition)  of  the  animal.  But  nothing  in  the 
character  of  the  response  suggests  that  negative  reaction 


Sensory  Discrimination:   The  Chemical  Sense    83 

to  a  chemical  stimulus  has  a  different  conscious  accom- 
paniment from  that  of  negative  response  to  a  mechanical 
stimulus.  The  most  natural  interpretation  of  them  all 
on  the  psychic  side  is  that  of  unpleasantness,  increasing 
in  intensity  as  the  reaction  takes  a  more  violent  form.1 
The  time  occupied  in  reacting  has,  however,  been  made 
a  basis  for  differentiating  the  response  to  different  chemi- 
cals. It  was  found  that  if  the  worms  were  suspended 
by  threads,  and  their  anterior  ends  dipped  into  solutions 
of  sodium,  ammonium,  lithium,  and  potassium  chlorides, 
the  animals  reacted  to  these  substances  with  diminishing 
promptness  in  the  order  just  given.  The  differences  in  re- 
action time  were  marked.  Now  all  four  of  these  substances 
produce  in  man  nearly  the  same  taste  quality,  salt,  for 
which  the  common  constituent  chlorine  is  therefore  held 
responsible.  The  sodium,  lithium,  ammonium,  and  po- 
tassium ions  have  apparently  but  little  effect  on  the 
human  taste  organs.  Since  the  earthworm  reacts  with  de- 
cided time  differences  to  the  four,  it  may  be  that  its  taste 
organs  are  specifically  affected  by  each,  and  that  different 
taste  qualities  may  be  occasioned  in  its  consciousness,  sup- 
posing it  to  be  conscious  (554).  Kribs  (410)  has  obtained 
evidence  of  localized  chemical  sensibility  in  the  annelid 
^Eolosoma;  weak  chemicals  would  produce  a  reaction 
only  if  applied  to  the  sensory  hairs  of  the  head  end.  Leeches 

1  W.  W.  Norman  argued  that  the  squirming  reactions  of  worms,  and 
the  corresponding  reactions  of  other  animals  to  injurious  stimulation,  can- 
not be  taken  as  evidence  of  an  accompaniment  of  disagreeable  conscious- 
ness, because  of  the  fact  that  when  the  worm,  for  instance,  is  cut  in  two, 
the  squirming  movements  are  confined  to  the  posterior  piece,  while  the  head 
end  crawls  away  undisturbed.  The  head  end,  he  urges,  containing  the 
cerebral  ganglia,  ought  to  be  the  part  capable  of  suffering,  but  it  gives  no 
reaction  (525).  We  cannot,  however,  conclude  from  the  absence  of  a  re- 
action under  abnormal  conditions  that  when  it  occurs  in  the  normal  state 
it  has  no  conscious  accompaniment. 


84  The  Animal  Mind 

seem  to  be  excited  to  their  feeding  reaction  by  a  combination 
of  mechanical  and  chemical  stimulation.  They  are  very 
sensitive  to  slight  water  disturbances,  and  react  by  stopping 
the  respiratory  movements  if  a  needle  is  touched  to  the 
surface  of  the  water  above  them.  Food  juice  diffused 
through  the  water  makes  them  very  active ;  while  in  this 
state  they  will  attach  themselves  to  a  glass  rod,  but  drop 
off  at  once.  When  they  attach  themselves  to  food  substance, 
however,  they  hold  on  with  traditional  tenacity.  Chemi- 
cals of  various  kinds  produce  withdrawing  reactions,  and 
Lohner  (438)  finds  evidence  that  leeches  experience  "  taste 
compensation."  When  we  have  to  eat  sour  fruit  we  can- 
cel the  sour  sensation  by  putting  sugar  on  the  fruit.  A 
five  per  cent,  sugar  solution  produced  withdrawing  reac- 
tions in  a  leech,  but  if  the  sugar  solution  was  mixed  with 
a  nine  per  cent,  salt  solution,  its  strength  had  to  be  raised  to 
seven  and  five  tenths  per  cent,  before  the  leech  reacted  to  it. 

§  19.    The  Chemical  Sense  in  Mollusks 

In  the  case  of  the  Mollusca  there  is  little  satisfactory 
evidence  on  the  subject  of  the  chemical  sense.  The 
Acephala,  to  which  the  clam,  oyster,  and  scallop  be- 
long, do  not  take  food  by  active  movements;  hence, 
of  course,  they  can  have  no  specific  feeding  reactions. 
Chemical  sensibility,  distributed  over  the  surface  of  the 
body,  has  been  observed  in  lamellibranchs,  a  branch  of 
the  Acephala  (522).  Gasteropods,  including  snails  and 
slugs,  have,  owing  to  their  active  food  taking,  more  use  for 
a  chemical  sense ;  in  marine  snails  it  seems  rather  definitely 
localized  in  the  feelers  (522).  Yung  found  in  the  snail 
Helix  pomatia  that  smell  was  most  acute  at  the  end  of  the 
feelers,  but  that  the  animal  even  when  deprived  of  its 


Sensory  Discrimination:   The  Chemical  Sense     85 

feelers  could  distinguish  perfume.  Taste  he  found  best 
developed  near  the  lips,  and  touch  sensibility  distributed 
over  the  body,  but  especially  toward  the  end  of  the  feelers 
(834,  835). 

Of  two  freshwater  snails,  Physa  and  Lymnaea,  the  latter, 
whose  movements  are  slower,  can  sense  food  at  a  greater 
distance  than  the  former.  In  Physa  an  interesting  rela- 
tion between  the  chemical  and  mechanical  stimulation 
produced  by  contact  with  food  is  apparent.  "If  Physa," 
says  Dawson  (177),  "was  moving  at  a  moderately  rapid 
rate  when  it  came  in  contact  with  the  meat,  it  received  a 
sufficiently  strong  stimulus  to  cause  it  to  turn  away,  to 
pause  and  then  turn  back.  It  would  seem  that  the  me- 
chanical stimulus  was  not  only  sensed  first  but  obeyed, 
and  then  the  chemical  stimulus  was  in  turn  sensed  and 
obeyed."  The  limpets  Patella  and  Calyptraea  respond 
to  the  neighborhood  of  non-irritating  oils  by  withdrawing 
reactions  (583).  Irritating  chemicals,  of  course,  are  not 
proper  olfactory  stimuli,  but  one  can  hardly  be  sure  that  a 
stimulus  which  like  oil  of  bergamot  would  be  non-irritating 
to  the  human  mucous  membrane,  is  non-irritating  also  to 
the  body  surface  of  an  animal.  Mollusks  in  general 
seem  to  have  chemical  sensitivity  distributed  all  over  the 
body  surface,  although  certain  regions  are  especially  sensi- 
tive. Pieron  (585)  finds  in  marine  snails  three  modes  of 
chemical  excitability:  an  aerial  distance  excitability,  on 
all  parts  of  the  body  with  predominance  of  the  mouth, 
the  anterior  edge  of  the  foot,  and  the  siphon ;  a  contact 
sensibility  in  both  air  and  water,  on  the  mouth,  the 
horns,  and  probably  elsewhere;  and  a  delicate  distance 
sensibility  in  the  water,  located  in  the  regions  of  the 
mouth,  the  horns,  the  anterior  edge  of  the  foot,  and 
the  osphradial  region. 


86  The  Animal  Mind 

§  20.    The  Chemical  Sense  in  Echinoderms 

In  the  phylum  of  the  echinoderms,  under  which  are 
classed  starfish  and  sea-urchins,  the  "circular  symmetry" 
of  body  structure  characteristic  of  the  ccelenterates  re- 
appears. Starfish  were  found  by  Romanes  many  years 
ago  to  show,  besides  pronounced  negative  reactions  to  strong 
or  injurious  mechanical  stimulation,  what  he  called  a  sense 
of  smell.  Its  manifestations  depended  on  the  physiological 
condition  of  the  animal ;  that  is,  upon  its  degree  of  hunger. 
If  kept  several  days  without  food,  a  starfish  would  Immedi- 
ately perceive  its  presence  and  crawl  toward  it.  "  More- 
over, if  a  small  piece  of  the  food  were  held  in  a  pair  of  forceps 
and  gently  withdrawn  as  the  starfish  approached  it,  the 
animal  could  be  led  about  the  floor  of  the  tank  in  any 
direction."  By  cutting  off  various  parts  of  the  rays, 
Romanes  found  that  "the  olfactory  sense  was  equally 
distributed  throughout  their  length" ;  and  he  also  showed 
that  the  ventral  and  not  the  dorsal  surface  of  the  body 
was  concerned,  by  varnishing  the  latter,  which  left  the 
reactions  unaffected,  and  by  observing  that  when  a  bit  of 
food  was  placed  on  the  back  it  remained  unnoticed  (642, 
pp.  321-322).  Preyer  reported  great  individual  differences 
in  the  responses  of  starfish  to  food  stimulation;  while 
certain  specimens  were  unmoved  by  the  neighborhood  of 
food,  an  individual  of  another  species  came  from  more 
than  six  inches  away  and  fell  upon  it  (617).  Whether 
the  unlikeness  of  behavior  was  due  to  the  species 
difference  or  to  a  difference  in  the  degree  of  hunger 
does  not  appear.  In  the  holothurian  Thyone  briareus 
feeding  movements  could  not  be  produced  by  external 
stimuli,  and  apparently  result  from  the  internal  state 
of  hunger  (565). 


Sensory  Discrimination:   The  Chemical  Sense     87 

§  21.   The  Chemical  Sense  in  Crustacea 

The  highest  invertebrate  animals  belong  to  the  phylum 
of  the  Arthropoda,  like  the  annelid  worms  in  their  segmented 
structure,  but  more  highly  organized  in  many  respects. 
The  body  of  a  typical  arthropod  consists  of  a  series  of  seg- 
ments, one  behind  another,  each  segment  with  a  pair  of 
appendages.  The  higher  an  arthropod  stands  in  the 
scale,  the  more  modification  and  differentiation  of  func- 
tion there  is  in  the  segments  and  appendages ;  the  former 
often  become  consolidated,  and  the  latter  become  modified 
for  swimming,  walking,  or  sensory  purposes.  The  lowest 
grand  division  of  the  Arthropoda  is  that  of  the  Crustacea. 

As  the  animals  of  this  group  are  covered  with  a  hard  out- 
side shell,  sensitiveness  to  touch  and  chemical  stimulation 
is  ordinarily  referred  to  certain  hairs  scattered  over  the 
body,  and  to  the  modified  appendages  of  the  anterior  seg- 
ments which  we  commonly  know  as  "  feelers,"  the  large 
and  small  antennae.  That  mechanical  contact  stimuli 
in  certain  Crustacea  give  rise  to  specialized  reactions 
is  evidenced  by  observations  on  the  hermit  crab.  This 
animal,  as  is  well  known,  has  acquired  the  instinct  of  tak- 
ing up  its  abode  in  empty  shells,  most  commonly  those  of 
some  gasteropod  mollusk.  When  wandering  about  in 
search  of  a  dwelling,  the  crab's  reactions  to  the  objects  it 
meets  show  adaptation  to  the  character  of  the  stimulus,  for 
it  will  not  investigate  a  glass  tube  or  ball ;  the  smooth  sur- 
face seems  not  to  be  the  adequate  stimulus  for  beginning  the 
movements  involved  in  exploring  and  entering  a  shell  (194). 

The  responses  of  Crustacea  to  food  stimulation  vary,  as 
might  be  expected,  with  different  genera  and  species. 
Nagel  finds  the  role  of  the  food  sense  in  aquatic  Crustacea 
very  insignificant ;  they  occasionally  show  antennal  move- 


88  The  Animal  Mind 

ments  in  the  presence  of  food,  he  says,  but  are  not  guided 
to  it  (522).  That  general  restlessness  is  shown  by  various 
Crustacea  in  the  neighborhood  of  food,  but  not  in  contact 
with  it,  has  been  observed  by  Bell  in  the  crayfish  (40),  by 
Holmes  in  the  amphipod  Amphithoe  longimana  (329),  by 
Bateson  in  shrimps  and  prawns  (24),  and  by  Bethe  in  the 
green  crab  (49).  Bethe  arranged  a  series  of  aquaria  one 
above  the  other,  with  a  connection  between  them,  and 
found  that  when  food  was  placed  in  the  uppermost  compart- 
ment the  crabs  in  the  lower  ones  were  successively  excited 
as  the  food  juices  diffused  themselves  from  each  compart- 
ment to  the  one  below.  In  the  amphipod  Amphithoe  longi- 
mana, the  small  antennae  and  the  mouth  parts  appeared  to 
be  the  regions  especially  sensitive  to  food  stimulation ;  if  the 
food  touched  one  of  the  former,  the  animal  instantly  made  a 
dart  for  it.  Touching  the  antennule  with  a  needle  very 
rarely  caused  such  a  reaction  (329).  Bateson's  shrimps  and 
prawns  had  their  food  sensibility  located  chiefly  in  the  an- 
tennules,  though  if  food  was  placed  very  near  them  they  would 
show  disturbance  even  when  deprived  of  antennules  (24). 
Balss  (15)  finds  the  sense  of  smell  in  the  shrimp  Palaemon 
located  in  the  antennae,  and  also  in  other  parts;  taste  in 
the  mouth  parts  and  tips  of  the  thoracic  legs.  This  was 
the  case  also  with  Holmes's  amphipod.  Bell,  on  the  other 
hand,  found  the  whole  body  of  the  crayfish  sensitive  to 
chemical  stimulation,  and  no  evidence  that  the  small 
antennae  were  especially  concerned.  The  crayfish's  reac- 
tions to  contact  with  food  were  such  as  to  direct  the  stimu- 
lus toward  the  mouth ;  negative  reactions  of  rubbing, 
scratching,  and  pulling  at  the  affected  part  were  obtained 
by  stimulation  with  acids,  salts,  and  other  irritants  (41). 
Chidester  (120)  found  that  the  crayfish  would  go  to  freshly 
cut  meat  more  quickly  than  to  meat  whose  surface  had  had 


Sensory  Discrimination:   The  Chemical  Sense    89 

time  to  dry.  Evidences  of  irritation  by  the  neighborhood  of 
asafcetida  were  observed  also  by  Graber  in  Pagurus  (268). 
In  some  Crustacea  the  sense  of  smell  is  possibly  con- 
cerned in  guiding  the  male  to  the  female.  Certain  cope- 
pods  which  daily  migrate  from  near  the  surface  of  the  water 
to  greater  depths  and  back  again  have  had  this  behavior 
explained  as  a  result  of  the  reactions  of  the  females  to  light, 
plus  the  tendency  of  the  males  to  follow  the  females.  That 
the  latter  is  an  affair  of  chemical  stimulation  is  indicated 
by  the  fact  that  the  females  were  sought  even  when  con- 
cealed in  tubes  (534).  In  the  case  of  some  other  Crustacea, 
however,  the  sexes  do  not  seem  to  be  aware  of  each  other's 
neighborhood  until  they  come  into  actual  contact  (331, 
333)- 

§  22.    The  Chemical  Sense  in  Arachnida 

The  two  most  important  divisions  of  the  phylum  Arthrop- 
oda,  besides  the  Crustacea,  are  those  of  the  Arachnida  and 
Insecta.  Spiders,  as  is  well  known,  have  highly  developed 
responses  to  mechanical  stimulation;  the  web-making 
species  in  particular  are  sensitive  to  very  slight  web  vibra- 
tions. The  food  reactions  of  spiders  have  never,  so  far 
as  the  writer  knows,  been  tested,  but  various  observers 
report  sensitiveness  to  chemical  stimulations,  such  as  those 
produced  by  odorous  oils,  not  in  contact  with  the  body. 
Spiders  of  the  family  Attidag  would  react  to  glass  rods 
dipped  in  such  oils  and  brought  close  behind  them,  but 
would  not  react  to  clean  glass  rods  when  similarly  placed 
(570).  The  reactions  seem  to  be  of  a  negative  character 
(618),  and,  of  course,  in  all  such  cases  it  remains  uncertain 
whether  the  possible  conscious  accompaniment  is  a  specif- 
ically olfactory  unpleasantness  or  an  unpleasant  irritation 
of  the  body  surface.  Pritchett  found  that  irritating  and 


90  The  Animal  Mind 

non-irritating  oils  gave  negative  reactions  (618) ;  but  an 
oil  that  belongs,  for  us,  to  the  latter  class  might  belong 
to  the  former  in  the  case  of  a  spider.  If  the  sensibility 
were  sharply  localized,  that  fact  would  point  in  the  direc- 
tion of  a  specific  olfactory  sensation;  but  while  some 
authorities  think  the  spider's  feelers  or  palpi  are  smell 
organs  (47),  others  believe  that  sensibility  to  chemical 
stimulation  is  distributed  over  the  body  (452,  618).  Nagel 
finds  no  specific  organ  of  smell  and  little  smell  sensibility 
in  spiders  (522). 

A  member  of  the  Arachnida  which  presents  but  slight 
superficial  resemblance  to  the  spiders  is  Limulus,  the 
horseshoe  crab.  Limulus  shows  taste  reactions,  but  no 
response  to  smell  stimuli.  If  the  mandibles  at  the  base 
of  the  legs  be  rubbed  with  inedible  objects,  there  is  no 
reaction.  Similar  negative  results  are  obtained  by  hold- 
ing strong-smelling  food  close  to  the  mouth  or  jaws.  But 
if  an  edible  substance  be  rubbed  on  the  mandibles,  strong 
chewing  movements  take  place.  Ammonia  or  acid  vapor 
will  produce  these  same  chewing  reflexes,  but  the  claws 
make  snapping  movements  "as  though  to  pick  away 
some  disagreeable  object."  If  a  wad  of  blotting  paper 
wet  with  ammonia  or  acid  be  laid  on  the  mandibles,  the 
chewing  movements  are  reversed  and  the  object  is  some- 
times picked  up  by  the  claws  and  removed.  Patten  found 
organs  which  he  believed  to  be  gustatory  on  both  the 
mandibles  and  the  claws  (557).  Pearl  observed  no  gusta- 
tory reactions  in  the  free-swimming  embryo  of  Limulus 

(562). 

§  23.    The  Chemical  Sense  in  Insects 

Throughout  all  the  branches  of  the  animal  kingdom  thus 
far  mentioned,  the  chemical  sense  has  functioned  chiefly  as 


Sensory  Discrimination:   The  Chemical  Sense     91 

a  food  sense.  There  has  been  but  little  evidence  of  the 
development  of  qualitative  discrimination  within  the  sense 
itself.  That  is,  while  in  many  cases  an  animal  can  appar- 
ently distinguish  the  edible  from  the  inedible,  and  gives 
negative  reactions  to  irritating  chemicals,  one  would  hardly 
be  justified  in  saying  that  it  possesses  more  than  one  food 
sensation  quality ;  while  in  our  own  case,  of  course,  though 
we  make  comparatively  little  use  of  the  sense  of  smell,  the 
qualitative  discriminations  possible  by  its  means  are 
many.  But  we  come  now  to  a  group  of  animals  where 
there  appears  a  remarkable  development  of  qualitative 
variety  in  the  sensations  resulting  from  chemical  stimula- 
tion; namely,  the  Insecta.  As  the  reactions  of  animals 
to  mechanical  stimulation,  on  the  other  hand,  offer  evi- 
dence of  little  qualitative  difference  in  the  accompanying 
sensations,  we  shall  give  but  slight  attention  to  them  in 
what  follows. 

To  begin  with,  there  is  evidence  that  taste  and  smell  are 
distinct  in  many  insects.  The  water  beetle  Dytiscus  mar- 
ginalis,  found  apparently  unresponsive  to  food  at  a  dis- 
tance, will  bite  with  especial  eagerness  at  filter  paper  soaked 
in  what  Nagel  calls  "a  pleasant  solution"  (522).  Ants 
fed  honey  mixed  with  strychnin  will  taste  it  and  then 
stop,  and  will  do  this  even  when  the  antennae  and  mouth 
palpi  are  removed,  indicating  that  the  taste  organs  are 
in  the  mouth  itself  (231).  Similar  results  have  been  ob- 
tained from  similar  tests  on  wasps,  and  it  has  been  observed 
that  wasps  so  treated  will  hesitate  when  offered  pure  honey 
afterward  (786). 

Essenberg  (208)  found  that  the  water  strider,  when 
offered  flies  which  had  been  soaked,  some  in  quinin  and 
alcohol,  some  in  coal  oil,  some  in  ammonia,  approached 
them  "  carefully,"  left  them,  and  then  returned  and  devoured 


92  The  Animal  Mind 

them,  a  proceeding  which  proved  fatal  in  certain  instances. 
The  insects  would  stop  and  retreat  just  before  reaching  a 
drop  of  coal  oil. 

Vitus  Graber  tested  the  reactions  of  various  insects  to 
odors  by  the  method  which  we  called  on  page  55  the 
Method  of  Preference.  This  was  Graber's  favorite  mode 
of  studying  the  effect  of  stimuli  upon  animals.  Applied 
to  olfactory  stimuli  it  consisted  in  offering  a  choice  between 
different  compartments,  containing  each  a  different  odor. 
The  animal's  power  of  discrimination  was  argued  from  the 
tendency  to  choose  certain  odors  rather  than  others. 
Such  preferences  were  shown  by  the  insects  (268).  The 
method,  however,  as  was  noted  above,  is  unsatisfactory, 
because  discrimination  might  exist  where  preference  did 
not.  Another  criticism  urged  against  Graber's  experi- 
ments is  that  the  odors  used  were  too  strong  and  irritating. 
There  is  always  the  possibility  that  such  substances  affect 
other  nerves  than  those  of  smell.  The  insects  observed  by 
Graber  displayed  choice  between  odors  even  when  their 
antennae  were  removed.  There  is  much  evidence  to  show 
that  the  antennse  are  the  true  organs  of  smell  in  insects. 
Various  flies  and  beetles  which  are  in  the  habit  of  laying 
their  eggs  in  putrefying  flesh  will  not  react  to  it  when  their 
antennae  are  removed,  and  it  has  been  shown  that  insects 
which  seem  to  find  their  mates  by  response  to  olfactory 
stimulation  fail  to  do  so  when  deprived  of  antennae  (231). 
Interesting  "compensatory  movements"  have  been  seen 
in  silkworm  moths  with  one  antenna  removed;  they 
turned,  that  is,  in  the  direction  of  the  remaining  antenna 
(397).  We  shall  note  movements  of  this  class  later  in 
insects  with  one  eye  blackened,  and  in  fish  with  one  audi- 
tory nerve  cut.  The  exploring  movements  of  the  antennae 
which  certain  insects  make  in  seeking  a  proper  place  to 


Sensory  Discrimination:   The  Chemical  Sense     93 

lay  their  eggs  have  been  taken  as  evidence  of  the  smell 
function  of  these  organs  (574).  Mclncjoo  (455,  456,  457, 
458),  however,  has  recently  presented  evidence  against 
the  olfactory  function  of  the  antennae.  His  experiments 
were  performed  on  beetles,  ants,  honey  bees,  and  hornets. 
His  line  of  argument  is  as  follows.  While  it  is  true  that 
insects  whose  antennae  have  been  removed  fail  to  respond 
normally  to  odors,  this  is  because  such  insects  are  abnormal 
in  all  their  behavior.  There  exist  in  various  regions  of 
the  body  of  insects,  as  for  instance  the  bases  of  the  wings 
and  legs,  small  pores  containing  sense  cells ;  these  Mclndoo 
calls  olfactory  pores.  He  finds  by  measuring  the  time 
required  for  insects  to  respond  to  odors,  that  this  reaction 
time  is  lengthened  more  decidedly  when  the  olfactory 
pores  are  varnished  over  than  when  the  antennae  are 
removed. 

The  function  of  the  chemical  sense  in  the  mating  processes 
of  insects  is  one  of  the  most  remarkable  phenomena  con- 
nected with  the  sensory  reactions  of  animals.  Forel  says 
he  had  a  female  Saturnia  moth  shut  up  in  his  city  room, 
and  that  within  a  short  time  a  number  of  males  came  and 
beat  against  the  window  (231).  Riley  hatched  in  Chicago 
some  moths  from  the  Ailanthus  silkworm,  which  were 
carefully  confined.  No  other  specimens  were  known  to 
exist  within  hundreds  of  miles.  A  virgin  female  was  put 
in  a  wicker  cage  on  an  ailanthus  tree,  and  a  male,  with  a 
silk  thread  tied  around  the  abdomen  for  identification, 
was  liberated  a  mile  and  a  half  away.  The  next  morning 
the  two  were  together  (637). 

The  most  interesting  observations  on  the  sense  of  smell 
as  used  in  the  mating  of  insects,  however,  are  those  of 
Fabre.  A  cocoon  of  the  "Bombyx  du  chene"  a  species  of 
which  Fabre  had  not  seen  a  specimen  in  the  locality  for 


94  The  Animal  Mind 

twenty  years,  was  brought  to  him,  and  from  it  a  female 
hatched.  Sixty  males  sought  her  within  a  few  hours  after 
she  reached  maturity.  Fabre  noticed  in  this  and  other 
cases  that  shutting  the  female  in  an  air-tight  box  prevented 
the  males  from  being  guided  to  her,  but  that  the  smallest 
opening  was  enough  to  allow  the  odor  to  escape;  that 
the  males  were  not  in  the  least  confused  or  led  astray  by 
placing  dishes  of  odorous  substances  about,  and  that  they 
would  seek  anything  on  which  the  female  had  rested  for 
a  time,  a  fact  which  suggests  that  the  stimulus  is  a  secre- 
tion of  the  body,  as  it  is  known  to  be  in  silkworm  moths. 
Fabre  offers  the  suggestion  that  smell  stimuli  as  they  are 
operative  in  the  animal  kingdom  generally  may  be  of  two 
classes :  (i)  substances  which  give  off  particles  in  vapor 
or  gas,  and  (2)  substances  which  give  off  a  form  of  vibra- 
tion. Our  own  olfactory  sense  is  limited  to  the  first  class 
of  stimuli,  but  some  animals,  notably  insects,  may  be 
sensitive  to  both  (216).  Certainly  the  marvellous  sensitive- 
ness involved  in  these  mating  reactions  suggests  a  kind  of 
response  to  stimulation  unknown  in  human  experience. 

§  24.   How  Ants  Find  Food 

In  many  ways  the  Hymenoptera  are  the  most  interesting 
of  insects,  particularly  those  members  of  the  order  which 
have  developed  community  life.  Their  reactions  to  chemi- 
cal stimulation  have  been  the  subject  of  a  large  mass  of 
literature,  some  of  the  more  important  results  of  which 
we  may  now  undertake  to  survey,  considering  ants,  bees, 
and  wasps  successively.  Sir  John  Lubbock  was  among 
the  earliest  observers  to  indicate  the  great  importance  of 
chemical  stimuli  in  the  life  of  ants.  In  the  first  place, 
he  demonstrated  that  it  is  by  chemical  stimulation  that 


Sensory  Discrimination:    The  Chemical  Sense     95 

ants  are  able  to  follow  each  other  to  supplies  of  food;  or  to 
larvae,  for  an  ant's  behavior  to  an  ant  larva  found  in  the 
course  of  its  wandering  is  like  its  behavior  to  food;  the 
larva  is  picked  up  and  carried  to  the  nest.  Lubbock  put 
some  larvae  on  a  glass  plate  at  a  little  distance  from  one  of 
his  artificial  ant  nests,  and  set  a  similar  empty  plate  beside 
it ;  he  then  made  a  bridge  of  a  strip  of  paper  leading  from 
the  nest  toward  the  plates,  and  connected  each  of  them 
with  this  bridge  by  a  separate  short  paper  strip.  He  placed 
a  marked  ant  at  the  larvae ;  she  picked  up  one  and  returned 
to  the  nest.  She  soon  appeared  followed  by  several  others ; 
when  she  had  reached  the  larvae,  and  before  the  others 
had  arrived  at  the  dividing  of  the  ways,  Lubbock  exchanged 
the  short  strips,  so  that  the  one  over  which  the  marked 
ant  had  passed  now  led  to  the  empty  plate.  The  following 
ants  all  took  this  path,  indicating  that  they  were  guided 
by  some  trace  which  her  footsteps  had  left.  Lubbock  was 
inclined  to  think,  however,  that  some  kind  of  communi- 
cation must  have  passed  between  the  marked  ant  and  her 
fellows  in  the  nest  to  induce  them  to  follow  her,  and  also 
that  this  communication  might  on  occasion  convey  some 
notion  of  the  quantity  of  food  or  larvae  to  be  had.  He 
placed  three  glass  plates  near  an  ant  nest,  connecting  each 
of  them  with  the  nest  by  means  of  a  paper  strip.  On  one 
plate  he  put  a  heap  of  several  hundred  larvae,  on  the  second 
two  or  three  only ;  the  third  was  empty.  He  put  a  marked 
ant  on  each  of  the  plates,  and  captured  all  the  ants  which 
they  led  back  with  them.  Many  more  ants  came  to  the 
plate  with  the  larger  heap  of  larvae  than  to  the  others. 
Lubbock  explained  this  by  supposing  that  the  ant  from  that 
dish  had  in  some  way  communicated  to  the  nest  the  greater 
numbers  at  her  disposal  (441,  pp.  172  ff.).  Obviously 
it  would  be  enough  to  suppose  that  the  smell  of  food  or 


96  The  Animal  Mind 

larvae  about  an  ant  returning  laden  to  the  nest  is  a  stimulus 
to  her  nest  mates  to  follow  her ;  that  this  smell  is  stronger, 
the  larger  the  stock  she  has  found,  and  hence  acts  as  a  more 
powerful  stimulus.  The  question  arises,  however,  as  to 
how  an  ant  can  distinguish  between  the  smell  of  food  or 
larvae  on  an  ant  that  has  just  found  a  store  of  either,  and 
the  smell  of  the  food  and  larvae  in  the  nest,  which  must  ad- 
here to  all  her  nest  mates.  Some  peculiarity  of  behavior  on 
the  part  of  the  foraging  ant  would  seem  to  be  needed  if 
she  is  to  induce  her  fellows  to  accompany  her  to  food. 
Wheeler,  whose  knowledge  of  ants  is  unsurpassed,  but  who 
is  perhaps  a  little  too  much  inclined  to  humanize  them, 
says  (783,  page  535),  "  I  believe  that  no  one  who  has  watched 
ants  continuously  and  under  a  variety  of  conditions  will 
doubt  that  they  actually  communicate  with  one  another. 
This  is  clearly  indicated  by  the  rapidity  with  which  they 
congregate  on  a  spot  where  one  of  their  number  has  found 
food,  or  retire  from  any  spot  in  which  a  few  of  their  num- 
ber have  been  killed  or  injured."  Such  communication, 
whatever  its  nature,  concerns  us  here  only  so  far  as  smell 
may  be  involved  in  responding  to  it. 

§  25.    The  Use  of  Smell  in  Path-finding  by  Ants 

The  homing  of  ants  is  a  puzzling  problem.  Bethe  (51) 
thinks  that  ants,  as  reflex  machines,  are  drawn  along  the 
path  back  to  the  nest  by  the  chemical  stimulus  deposited 
on  the  path  by  their  own  bodies.  Pieron  (579)  has  main- 
tained that  in  some  species  we  have  to  do  with  a  kind  of 
muscular  memory,  the  ants  simply  reversing,  on  the  home- 
ward path,  all  the  turnings  they  took  on  the  way  out,  like 
a  top  unwinding  itself.  Cornetz  (143,  144,  145)  claims 
for  ants  the  mysterious  power  of  registering  in  their  bodies 


Sensory  Discrimination:   The  Chemical  Sense    97 

the  general  direction  of  their  outward  course  and  reversing 
it  when  they  have  found  a  load  to  be  carried  home.  We 
may  consider  very  briefly  the  facts  that  have  been  brought 
to  the  support  of  these  various  hypotheses.  There  are 
really  two  problems  involved  in  the  homing  of  ants.  There 
is,  first,  the  problem  of  the  homing  of  a  solitary  forager, 
who,  having  found  food  at  the  end  often  of  a  very  long  and 
rambling  course,  is  able  to  get  back  to  the  nest.  Secondly, 
there  is  the  problem  of  the  nature  of  a  frequented  ant  road, 
along  which  many  ants  constantly  travel  to  and  from  the 
nest.  The  evidence  that  smell  functions  in  the  homing 
process  is  strongest  in  the  case  of  such  a  frequented  trail. 
Lubbock's  experiments  showed  that  on  these  trails  the 
recognition  of  visual  landmarks  plays  no  important  part. 
For  instance,  he  placed  larvae  in  a  dish  on  a  table  connected 
by  a  bridge  with  an  ant  nest.  He  accustomed  the  ants  to 
go  back  and  forth  between  the  dish  and  the  nest  by  a  path 
which  he  diversified  with  artificial  scenery,  such  as  rows  of 
bricks  along  either  side,  and  a  paper  tunnel.  When  the 
path  was  thoroughly  learned,  he  moved  the  bricks  and  the 
tunnel  so  that  they  led  in  a  different  direction :  the  ants, 
however,  were  not  at  all  disconcerted  by  this  cataclysm  of 
nature,  but  followed  the  same  track  as  before,  evidently 
guided  by  their  own  footprints  (441,  p.  259).  Forel  (233) 
showed  that  when  a  piece  of  wood  is  laid  across  a  well- 
frequented  path  of  certain  species  of  ants,  they  are  much 
disturbed  and  at  a  loss  to  follow  the  trail,  and  Bethe  (51) 
reports  that  drawing  a  finger  across  the  trail  will  apparently 
break  its  effectiveness  as  a  guide.  That  the  chemical 
deposited  by  the  ants  is  volatile  he  concludes  from  the  fol- 
lowing observation.  If  a  strip  of  paper  be  placed  across 
an  ant  path,  the  ants  on  coming  to  it  stop,  quest  about,  and 
are  delayed  until  one  accidentally  runs  across  the  strip 


98  The  Animal  MM 

and  others  follow.  The  piece  of  paper  is  thus  gradually 
adopted  into  the  ant  road ;  if  it  is  subsequently  removed, 
the  ants  stop  and  are  bewildered  at  the  place  where  it  was, 
showing  that  the  earlier  traces  of  their  footsteps,  under 
the  paper,  have  evaporated.  Again,  Bethe  thinks  he  has 
evidence  that  the  chemical  stimulus  left  by  the  feet  of 
ants  going  from  the  nest  is  different  from  that  deposited 
by  those  going  to  the  nest,  and  that  ants  on  the  way  home 
will  not  follow  a  track  made  by  the  feet  of  other  ants  on 
the  outward  journey,  and  vice  versa  (51).  Bethe  found 
that  when  the  usual  road  to  an  ant  nest  had  been  inter- 
rupted by  the  removal  of  a  heap  of  sand,  and  the  road 
across  the  breach  had  been  established  solely  by  incoming 
ants,  the  outgoing  ants  refused  to  follow  it,  and  made  a 
new  road  for  themselves  (51).  Wasmann  thinks  this  may 
have  been  done  merely  on  account  of  the  faintness  of  the 
recently  established  path  as  compared  with  the  old  one 
(762).  Bethe  observed  also  that  if  a  strip  of  paper  had 
been  adopted  into  an  ant  road,  and  was  then,  while  an  ant 
was  on  it,  rotated  through  180  degrees,  the  ant  stopped 
and  was  disturbed  on  coming  to  the  end  of  it  (51).  Experi- 
ments on  rotating  ants  were  made  also  by  Lubbock  (441), 
and  seem  to  give  puzzling  and  conflicting  results ;  it  is  not 
clear  why,  even  on  the  assumption  that~ there  is  a  difference 
in  odor  between  the  road  to  the  nest  and  that  from  the 
nest,  an  ant  on  a  road  which  led  both  ways  should  have 
found  her  course  interrupted  by  rotation.  One  fact,  Bethe 
thinks,  shows  that  even  assuming  two  road  smells  is  not 
enough.  Ants  of  certain  families  (Lasius)  which  habitually 
make  regular  and  frequented  roads  can,  if  they  come  upon 
one  of  these  roads  in  wandering,  at  once  take  the  proper 
direction,  either  to  or  from  the  nest.  Evidently  the  mere 
presence  of  two  smells  would  not  enable  them  to  do  this. 


Sensory  Discrimination:   The  Chemical  Sense     99 

Bethe  suggests  that  the  particles  of  the  two  chemical  sub- 
stances are  also  differently  polarized,  so  that  one  of  them 
can  be  followed  only  in  one  direction,  the  other  in  the  oppo- 
site direction  (51).  Wasmann  objects  to  this  that  an  ant 
returning  on  its  own  traces  would  destroy  them,  as  the 
opposite  polarizations  would  cancel ;  and  that  similar  con- 
fusion would  occur  on  a  narrow  and  much  frequented 
road  (762).  He  and  Forel  (233)  both  think  that,  granting 
the  discrimination  between  the  outward  and  inward  paths, 
which  is  made  by  only  a  few  families  of  ants,  the  direction 
is  most  probably  given  by  a  perception  obtained  through 
the  antennae,  of  the  " smell  form"  of  the  footsteps. 
Since  the  antennae  are  movable  organs,  like  the  hands, 
they  may  well,  Forel  suggests,  mediate  spatial  perceptions 
of  the  form  and  size  of  odorous  patches.  This  hypothesis 
would  fall  to  the  ground  if  Mclndoo's  contention  that 
the  antennas  are  not  smell  organs  were  sustained. 

On  the  whole,  there  is  much  evidence  indicating  that 
smell  plays  an  important  part  in  determining  the  response 
of  ants  to  well-frequented  roads.  We  may  now  consider 
the  case  of  the  solitary  forager.  Santschi  (654)  believes 
that  he  has  seen  a  smell  trail  "  intentionally  "  deposited  by 
an  ant,  dragging  her  abdomen  along  the  path.  Bethe,  whose 
general  position  that  ants,  and  indeed  all  invertebrate  ani- 
mals, are  reflex  machines  requires  him  to  avoid  any  hypothe- 
sis that  would  involve  learning  or  memory  on  the  part  of 
these  animals,  is  of  course  anxious  to  explain  the  homing 
of  the  solitary  foraging  ant  as  a  smell  reflex.  He  placed 
near  the  entrance  of  a  nest  a  large  sheet  of  paper  covered 
with  lampblack,  on  which  the  footsteps  of  the  ants  could 
be  traced.  On  this  paper  he  put  a  supply  of  food.  When 
an  ant  had  found  the  food,  Bethe  reports  that  in  returning 
to  the  nest  she  always  followed  the  path  by  which  she  had 


ioo  The  Animal  Mind 

come,  except  that  when  the  original  path  had  crossed  itself 
in  loops,  th£  ant  omitted  the  loops  in  her  homeward  way 
(51).  Apparently,  however,  many  species  of  ants  do  not 
thus  retrace  their  own  footsteps.  The  l '  muscular  memory  " 
theory  of  Pieron  (579)  is  based  on  the  observation  that  if 
a  homing  ant  be  carefully  lifted  and  deposited  at  a  little 
distance  away,  she  will  continue  her  course  until  she  has 
traversed  a  distance  equal  to  that  which  she  would  have 
had  to  go  to  reach  her  nest,  if  her  course  had  not  been  inter- 
rupted. Cornetz's  theory  (143,  144,  145),  that  an  ant 
has  some  mysterious  power  of  retaining  an  impression  of 
the  direction  in  which  she  set  out,  and  of  reversing  this 
direction  when  she  is  ready  to  return  home,  is  derived  from 
a  long  series  of  very  careful  field  studies.  He  reports 
that  a  foraging  ant  takes  a  certain  general  direction  and 
makes  excursions  to  right  and  left  in  search  of  food.  When 
the  food  has  been  discovered,  she  reverses  her  original 
direction,  but  does  not  actually  retrace  any  part  of  her 
outgoing  path.  Pieron  (591)  is  impressed  by  these  obser- 
vations, and  inclined  to  think  that  a  mysterious  factor  is 
actually  involved.  That  the  direction  of  the  light  may 
serve  as  a  guide  in  the  homing  of  ants  is  indicated  by  obser- 
vations of  Lubbock  (441),  Turner  (722  a),  and  Santschi 
(654),  but  the  ability  of  ants  to  find  their  way  about  in  the 
dark  is  sufficient  proof  that  it  cannot  be  the  sole  factor. 

§  26.   How  Ants  "Recognize"  Nest  Mates 

Another  problem  of  ant  life  to  which  smell  appears  to 
furnish  the  key  is  that  of  the  recognition  of  nest  mates. 
It  has  long  been  known  that  an  ant  entering  a  strange 
nest,  though  of  the  same  species,  is  likely  to  meet  with 
rough  treatment,  and  even  be  put  to  death.  Now  Forel 


Sensory  Discrimination:   The  Chemical  Sense    101 

found  in  1886  that  ants  of  the  genus  Myrmica  whose  anten- 
nae were  removed  would  attack  their  own  nest  mates  (231). 
It  seems  probable  that  each  nest  of  ants  has  a  peculiar 
odor  which  is  the  basis  of  the  distinction  between  friends 
and  foes.  Be  the  tested  the  smell  theory  by  dipping  an 
ant  first  in  weak  alcohol,  then  in  water,  and  then  in  the 
juices  obtained  by  crushing  the  bodies  of  a  number  of  ants 
of  another  species.  He  found  that  an  ant  thus  treated 
would  be  attacked  and  killed  by  its  own  nest  mates,  but 
could  be  introduced,  though  not  so  easily,  into  the  nest 
whose  odor  it  now  presumably  bore,  even  though  its  ap- 
pearance was  quite  different  from  that  of  the  ants  therein 
(51).  Wasmann  repeated  these  experiments  with  much 
less  success  than  Be  the ;  bathing  Myrmica  ants  with  essence 
of  Tetramorium  ant  did  not  preserve  them  from  final  de- 
struction at  the  jaws  of  the  latter,  though  it  delayed  their 
fate  ;  nor  did  much  bathing  with  foreign  nest  odors  induce 
the  ants  to  attack  beetles  of  the  species  Lomechusa  strumosa, 
their  accustomed  " guests"  in  the  nest,  though  they  seemed 
disturbed  at  first.  Wasmann  apparently  thinks  other 
factors  besides  smell,  vision  perhaps,  enter  into  the  recogni- 
tion process  (762).  Bethe,  in  a  later  paper,  suggests  that 
Wasmann's  negative  results  may  have  been  due  to  the 
fact  that  the  nest  smell  very  quickly  returns  to  the  ants 
after  it  has  been  removed;  he  himself  took  account  only 
of  the  first  reaction  of  other  ants  toward  the  one  subjected 
to  treatment  (52).  Pieron  (581  a)  has  repeated  Bethe's 
experiments  and  confirmed  his  results  with  eighteen  dif- 
ferent combinations  of  ant  species.  Many  factors,  how- 
ever, modify  the  hostile  reaction  to  foreigners.  Pieron 
finds  that  certain  species  are  inclined  to  be  tolerant.  At- 
tacks are  more  frequent  near  the  nest  than  at  a  distance 
from  it.  A  solitary  ant  tends  to  run  away  rather  than  to 


102  The  Animal  Mind 

attack.  Males  do  not  distinguish  strangers  from  nest 
mates,  and  a  female  after  the  marriage  flight  will  be  re- 
ceived in  a  strange  nest.  Brun  (104)  has  observed  that 
ants  carrying  larvae  will  be  tolerantly  received,  and  that  if 
ants  from  two  nests  are  tumbled  into  a  sack  together  and 
then  tumbled  out  into  a  strange  place,  their  hostility  to 
each  other  is  inhibited  by  their  general  disturbance  and 
fright. 

Termites,  which,  although  they  belong  to  the  order  of 
neuropterous  insects  and  not  to  the  Hymenoptera,  have 
developed  an  organized  community  life  much  like  that  of 
ants,  show  the  same  tendency  as  ants  to  attack  strangers. 
The  young  are  not  attacked,  nor  does  the  righting  response 
occur  when  large  numbers  are  hastily  tumbled  together. 
That  the  hostile  response  is  made  to  a  chemical  stimulus, 
at  least  in  part,  appears  from  the  fact  that  "a  well-washed 
termite  is  attacked  by  both  aliens  and  fellows,"  but  the 
observations  do  not  give  quite  so  definite  results  as  those 
on  ants  (6). 

Fielde,  as  the  result  of  a  study  of  the  genus  Stenamma, 
concludes  that  each  ant  is  the  bearer  of  three  distinct  odors : 
the  individual  odor,  which  enables  her  to  follow  her  own 
trail  in  a  labyrinth,  and  the  reception  of  which  depends  upon 
the  tenth  segment  of  the  antennae ;  the  race  odor,  depen- 
dent on  the  eleventh  segment ;  and  the  nest  odor,  depen- 
dent on  the  twelfth  (219).  No  other  investigator,  however, 
finds  evidence  of  any  such  specialization  of  the  antennal 
segments,  and  Mclndoo,  as  we  have  seen,  wholly  rejects 
the  antennae  as  a  smell  organ.  In  a  later  article  Fielde  con- 
cludes that  the  nest  odor  of  the  worker  ants  is  derived 
from  their  queen  mother;  that  the  odor  of  the  queen  is 
unchanging,  and  is  imparted  to  her  eggs.  The  worker, 
however,  gradually  changes  its  odor.  Queens  of  diverse 


Sensory  Discrimination:   The  Chemical  Sense     103 

odors  may  be  produced  by  the  influence  of  males  that  are 
the  offspring  of  worker  mothers  and  have  the  differentiated 
worker  odor.  A  young  ant  isolated  from  the  pupa  stage 
until  many  days  old  will  single  out  its  queen  mother  from 
queens  of  other  species,  but  will  show  decided  suspicion 
of  older  sister  worker  ants.  A  mixed  nest  formed  of  newly 
hatched  ants  of  different  species  was  separated  for  seven 
months.  On  rejoining  each  other,  the  ants  showed  hos- 
tility ;  their  odor,  Fielde  argues,  had  changed.  But  young 
ants  of  one  species  were  received  by  those  of  the  other 
species.  Fielde  does  not  hesitate  to  introduce  the  psychic 
factor  and  say  that  the  latter  remembered  the  odor  of  the 
young  ones,  having  been  associated  with  it  in  their  own 
youth.  The  suggestion  might  be  made  that  the  young 
ants  had  not  as  yet  developed  any  specific  odor,  but  this 
is  opposed  by  the  observation  that  newly  hatched  Lasius 
ants  from  a  strange  colony  were  not  received  by  a  nest 
of  Stenammas,  while  young  Lasius  ants  from  a  colony  with 
which  the  Stenammas  had  been  acquainted  in  youth  were 
accepted  eleven  months  after  the  latter  had  been  segregated. 
It  is  an  affair  of  the  memory,  Fielde  is  assured ;  and  she  says, 
"  If  an  ant's  experience  be  narrow,  it  will  quarrel  with  many, 
while  acquaintance  with  a  great  number  of  ant  odors  will 
cause  it  to  live  peaceably  with  ants  of  diverse  lineage,  pro- 
vided the  odors  characterizing  such  lineage  and  age  environ 
it  at  its  hatching  "  (2 24) .  Bethe  held  that  an  ant's  own  nest 
odor  offered  no  stimulus  to  it  at  all,  but  that  fighting  reflexes 
were  occasioned  by  any  foreign  nest  odor  (51).  Many 
facts,  however,  seem  to  tell  against  this  view;  among 
others,  the  early  observation  of  Forel  that  a  Myrmica 
ant  deprived  of  its  antennae  attacks  everything  in  sight 
(231).  It  should,  according  to  Bethe's  theory,  live 
peaceably  with  all. 


104  The  Animal  Mind 

Thus  we  see  that  in  spite  of  some  divergence  of  testi- 
mony, there  is  evidence  that  ants  have  a  variety  of  quali- 
tatively different  smell  experiences :  the  smell  of  food  and 
of  larvae,  probably  distinct,  though  there  is  no  experimental 
proof  of  the  fact;  the  individual  smell  of  an  ant's  own 
footsteps ;  a  possible  distinction,  in  some  species,  between 
the  smell  of  the  outgoing  and  that  of  the  incoming  paths ; 
and  the  different  odors  which  seem  to  be  responsible  for 
the  discrimination  between  nest  mates  and  foreigners. 
If  it  were  true,  as  Fielde  maintains,  that  loss  of  the  eighth 
and  ninth  segments  of  the  antennae  renders  an  ant  incapable 
of  caring  for  the  young,  then  the  recognition  of  larvae  and 
pupae  would  depend  upon  a  specific  odor  (219). 

§  27.  How  Bees  are  Attracted  to  Flowers 

In  bees  the  sense  of  smell  is  equally  well  developed.  But 
no  topic  in  comparative  psychology  has  been  more  hotly 
disputed  than  the  use  which  bees  make  of  this  sense,  and 
the  extent  to  which  they  depend,  rather,  upon  sight.  Dar- 
win (170)  and  H.  Mliller  (512,  513)  thought  both  color  and 
fragrance  influential  in  attracting  insects  to  flowers.  Plateau 
maintains  that  the  chief  influence  guiding  bees  to  flowers 
is  smell,  and  that  color  has  little  effect.  He  made  a  num- 
ber of  experiments  in  which  the  brightly  colored  corollas 
of  flowers  were  cut  off  without  disturbing  the  nectaries, 
and  claims  to  have  found  that  the  visits  of  bees  to  the  muti- 
lated flowers  were  as  frequent  as  before  (600-603,  605). 
On  the  other  hand,  Giltay  obtained  opposite  results ;  the 
flowers  whose  corollas  were  removed  were  neglected  by 
bees,  while  those  which  were  covered  so  as  to  be  invisible 
but  not  so  as  to  prevent  the  odor  from  escaping,  were  also 
unnoticed  (259).  Josephine  Wery  found  that  the  propor- 


Sensory  Discrimination:   The  Chemical  Sense    105 

tion  of  bees  visiting  flowers  with  intact  corollas  to  those 
visiting  flowers  with  the  corollas  removed  was  66 : 18 
(778).  Kienitz-Gerloff  criticises  Plateau's  figures  and  the 
accuracy  of  his  experiments  (400).  Forel  found  that  a 
bee  with  the  antennae  and  all  the  mouth  parts  removed, 
hence  probably  incapable  of  smell,  returned  to  flowers 
for  honey,  though  of  course  without  success  (231).  An- 
dreae  thinks  that  among  diurnal  insects  those  which  live 
on  the  ground,  and  take  but  short  flights,  are  more  influ- 
enced by  smell ;  while  the  freely  flying  insects  are  attracted 
by  the  sight  of  flowers  (5).  On  the  whole,  inconspicuous 
flowers  are  more  often  f ertilized  by  wind  than  by  the  visits 
of  insects. 

§  28.  How  Bees  Find  the  Hive 

Most  complicated  of  all  is  the  problem  as  to  how  bees  find 
their  way  back  to  the  hive.  It  is  obvious  that  the  simple 
ant  method  of  following  a  chemical  trail  is  ruled  out  for  in- 
sects that  fly.  Bethe  abandons  the  puzzle  as  insoluble  (51). 
Von  Buttel-Reepen  attempts  at  length,  and  with  a  vast 
amount  of  apic  lore,  to  refute  his  position.  It  would  be  im- 
possible to  give  more  than  the  briefest  statement  of  the 
arguments  of  both  sides.  Bethe  maintains  that  the  smell 
of  the  hive  does  not  guide  the  bees  back  to  it,  because  he 
found  that  if  the  hive  were  rotated  slowly  enough  to  allow 
the  cloud  of  nest  smell  at  the  opening  to  move  with  the 
opening,  the  bees  returning  would  not  follow  it  for  more 
than  45°,  but  would  go  to  the  place  where  the  opening 
had  been.  He  thinks  they  are  not  guided  by  sight,  be- 
cause when  he  completely  changed  the  appearance  of  the 
hive,  masking  it  with  branches  and  other  coverings,  the 
bees  were  not  disconcerted,  but  flew  straight  to  the  mouth 
of  the  hive.  He  brings  other  evidence  against  the  vision 


io6  The  Animal  Mind 

hypothesis  which  we  shall  discuss  in  Chapter  XI.  An 
unknown  force,  he  concludes,  guides  the  bee  in  its  homing 
flight  (51).  Von  Buttel-Reepen  believes  that  visual  mem- 
ory will  explain  all  the  facts ;  that  the  bees  were  not  dis- 
turbed by  the  altered  appearance  of  their  hive  because 
they  knew  their  way  so  thoroughly  that  nothing  could 
disturb  them  by  the  time  they  had  come  so  nearly  home. 
The  visual  memory  required  is,  he  admits  of  a  peculiar 
sort,  which  we  shall  consider  in  a  later  chapter.  The  odor 
of  the  hive  does  cooperate  with  vision  in  certain  cases; 
when  a  stock  of  bees  has  been  moved  without  their  knowl- 
edge, they  fly  out  without  making  any  " orienting  flight," 
as  they  commonly  do  on  leaving  a  new  place,  a  fact  that 
is  one  of  the  evidences  for  the  visual  memory  theory. 
Nevertheless,  many  of  them  succeed  in  finding  their  way 
back,  and  then,  if  their  hive  is  placed  among  a  number  of 
others,  von  Buttel-Reepen  thinks  they  " smell"  their 
way  back  to  the  right  one.  He  mocks  at  Bethe's  unknown 
force,  on  the  ground  that  it  must  sometimes  lead  the  bee 
to  the  hive  and  sometimes  back  to  the  place  where  food 
has  been  found  (115).  Bethe  attempts  to  answer  this  by 
saying  that  the  force  acts  in  cooperation  with  the  physio- 
logical condition  of  the  animal;  the  laden  bee  follows  it 
to  the  hive,  the  bee  with  the  empty  crop  is  led  back  to  the 
food  supply  (52).  Of  course  one  may  say  what  one  pleases 
about  the  modus  operandi  of  an  unknown  force  without 
fear  of  disproof,  but  also  without  carrying  much  conviction. 
That  a  mysterious  sense  of  direction  exists  in  the  bee 
is  concluded  by  Bonnier  (97)  from  the  following  evidence. 
He  first  showed  that  bees  whose  eyes  had  been  covered 
by  pigmented  collodion  could  go  directly  to  their  hive  if 
they  were  not  more  than  three  kilometers  away.  Smell, 
however,  or  muscular  memory,  might  account  for  this. 


Sensory  Discrimination:   The  Chemical  Sense     107 

He  then  attempted  to  demonstrate  that  smell  was  not 
an  essential  factor  in  guiding  bees.  He  placed  two  stands 
carrying  honey,  one  200  meters,  the  other  six  meters  from 
the  hive,  and  marked  the  bees  that  visited  each  stand, 
proving  that  a  given  bee  almost  never  went  to  both,  but 
continued  to  visit  the  stand  where  it  had  first  found  honey. 
Here,  however,  sight  might  have  been  the  determining  influ- 
ence. Wagner  (751)  thinks  that  bees  in  the  neighborhood 
of  the  hive  are  influenced  by  visual  landmarks,  but  that  in 
their  longer  flights  they  depend  on  a  sense  of  direction,  which 
seems  however  to  be  a  form  of  visual  memory.  On  the 
whole,  smell  would  appear  to  be  only  one  factor,  and  not 
a  very  important  one,  in  guiding  the  flights  of  bees. 

§  29.  How  Bees  "Recognize"  Nest  Mates 

The  nest  smell,  which  characterizes  each  hive  and  pre- 
vents the  reception  of  strangers,  who  are  treated  precisely 
as  by  ants  in  similar  circumstances,  is  composed  according 
to  von  Buttel-Reepen  of  the  following  odors:  the  indi- 
vidual odor  of  different  workers ;  the  family  odor,  common 
to  all  the  offspring  of  the  same  queen ;  the  larval  smell  and 
food  smell ;  the  drone  smell,  the  wax  smell,  and  the  honey 
smell.  There  are  various  ways  in  which  the  mode  of  reac- 
tion to  a  foreign  nest  smell  is  modified.  If  two  bee  stocks 
are  placed  side  by  side,  and  one  has  the  queen  and  entire 
brood  removed,  it  will  go  over  to  the  other  stock  and  be 
kindly  received.  One  can  understand  that  the  attraction 
of  the  queen  and  brood  odor  may  overcome  the  tendency 
of  the  foreign  nest  smell  to  repel  the  invaders,  but  it  is 
harder  to  see  why  the  more  fortunate  stock  should  allow 
itself  to  be  invaded.  Further,  a  bee  laden  with  honey 
can  get  itself  received  by  a  foreign  stock  that  has  exchanged 


io8  The  Animal  Mind 

hives  with  it,  where  an  unladen  bee  is  attacked ;  here  the 
smell  of  the  honey  may  overcome  the  foreign  smell.  As  is 
well  known,  two  alien  stocks  may  be  united  by  sprinkling 
them  with  some  odorous  substance.  The  queen  odor  is 
the  strongest  factor  in  the  nest  smell ;  in  swarming  it  over- 
comes the  tendency  to  return  to  the  old  nest,  and  queen- 
less  swarms  will  join  themselves  to  foreign  swarms  having  a 
queen.  The  apparent  attention  paid  to  the  queen  while 
laying  eggs,  the  gathering  of  workers  around  her  trilling 
their  antennae  toward  her,  suggest  strongly  that  her  odor  is 
pleasant  to  them.  The  queen,  herself,  however,  is  per- 
fectly indifferent  to  any  foreign  nest  smell,  and  will  beg 
food  of  any  bee,  even  those  which  are  angrily  crowded 
around  her  cage  in  a  foreign  hive.  Drones  also  will  go  from 
stock  to  stock,  and  are  always  peacefully  received  until 
drone-killing  time  begins.  It  has  usually  been  supposed 
that  the  unrest  displayed  by  a  bee  stock  when  deprived  of 
its  queen  is  due  to  the  absence  of  the  queen  odor,  and  it 
seems  almost  certain  that  this  must  be  a  powerful  influence, 
though  von  Buttel-Reepen  thinks  it  is  not  the  only  influ- 
ence, for  he  has  observed  that  if  the  queen  be  replaced  in 
the  honey  space,  removed  from  the  rest  of  the  hive,  the 
bees  will  quiet  instantly,  before  the  smell  has  had  time  to 
diffuse  itself.  Also,  bees  sometimes  behave  as  if  they  had 
lost  their  queen  when  she  is  only  put  in  a  cage,  and  her  odor 
is  perfectly  accessible  (115). 

It  is  clear  that  bees  as  well  as  ants  are  capable  of  dis- 
tinguishing a  considerable  number  of  smell  qualities.  Prob- 
ably the  same  thing  is  true  of  the  social  wasps.  In  the 
solitary  wasps,  however,  we  find  less  evidence  of  a  highly 
developed  sense  of  smell,  or  rather  of  a  great  variety  of  smell 
reactions,  and  the  solitary  bees  are  very  likely  less  influenced 
by  smell  than  the  social  bees.  In  the  interesting  study  of 


Sensory  Discrimination:   The  Chemical  Sense    109 

the  solitary  wasps  by  Mr.  and  Mrs.  Peckham,  it  appears 
that  sight  plays  a  far  more  important  role  than  smell  for 
these  insects,  and  the  return  to  the  nest  in  particular  seems 
to  be  almost  entirely  an  affair  of  sight  (572,  573).  In  gen- 
eral, the  greatest  development  of  qualitative  variety  in  the 
sense  of  smell  is  found  in  the  social  Hymenoptera,  and  is 
probably  a  product  of  the  social  state.  Ferris,  however, 
noted  that  the  solitary  wasp  Dinetus  was  much  disturbed 
in  finding  its  nest  hole  if  he  had  placed  his  hand  over  the 
hole  during  the  wasp's  absence,  and  thought  the  odor  of 
his  hand  was  distracting  to  the  insect  (574). 


§  30.    The  Chemical  Sense  in  Vertebrates 

Although  the  vertebrates  stand  at  the  head  of  the  animal 
kingdom,  yet  in  point  of  complexity  of  structure  and  behav- 
ior the  lowest  vertebrate  is  far  below  the  highest  members 
of  the  invertebrate  division.  When  we  undertake  to  study 
the  responses  to  special  stimulation  displayed  by  this 
same  lowest  vertebrate,  the  little  Amphioxus  or  lancelet, 
it  is  like  going  back  to  the  earthworm.  The  only  kind  of 
evidence  that  contact,  chemical,  and  temperature  stimuli 
produce  specific  sensation  qualities  is  found  in  the  fact  that 
sensibility  to  them  is  differently  localized,  and  may  be  in- 
dependently fatigued.  To  weak  acid,  the  head  end  of  the 
animal  is  most  sensitive,  the  posterior  end  less,  the  middle 
least;  to  contact  with  a  camePs-hair  brush,  the  two  ends 
are  equally  sensitive  and  more  so  than  the  middle;  to  a 
current  of  warm  water  the  order  of  sensitiveness  is :  head 
end,  middle,  posterior  end  (541). 

For  fishes,  as  for  all  aquatic  animals,  the  distinction  be- 
tween smell  and  taste  becomes  obscure.  The  neighborhood 
of  food  not  in  actual  contact  with  the  body  seems  to  stir 


no  The  Animal  Mind 

fish  to  activity,  but  not  to  direct  their  movements.  Bateson 
(25)  and  Herrick  (297)  both  obtained  evidence  of  this; 
Nagel,  on  the  other  hand,  declares  that  fish  do  not  perceive 
food  at  a  distance  except  by  sight,  and  that  the  function 
of  the  first  pair  of  cranial  nerves  in  these  animals  must 
remain  uncertain  (522).  The  well-developed  character 
of  these  " olfactory"  nerves  and  lobes,  whose  function  in 
higher  vertebrates  is  certainly  connected  with  smell,  would 
argue  against  the  supposition  that  smell  can  be  wholly 
lacking  in  fishes.  It  is  generally  agreed  that  a  contact 
food  sense  exists  in  fish;  Nagel,  however,  holds  that  its 
organs  are  situated  only  about  the  mouth  (522),  while 
Herrick  has  good  experimental  proof  that  fishes  which 
have  "  terminal  buds,"  structures  resembling  taste  buds, 
distributed  over  the  skin,  are  also  sensitive  to  food  stimu- 
lation applied  to  different  regions  of  the  skin.  He  thinks 
that  Nagel's  negative  results  were  due  to  the  fact  that 
instead  of  food  stimuli  in  his  experiments  he  used  chemicals 
with  which  the  fish  would  not  normally  be  acquainted  (297). 
Parker  (546),  experimenting  with  catfish  and  the  young 
of  a  species  of  lamprey,  found  the  whole  body  surface  more 
or  less  sensitive  to  salt,  acid,  and  alkali;  the  body  of  the 
lampreys  was  sensitive  also  to  quinin  solution,  but  that 
of  the  catfish  was  not;  neither  animal  displayed  skin 
sensitiveness  to  sugar  solution.  Cutting  the  nerve  supply 
to  the  olfactory  organs,  the  lateral-line  organs  (see  page 
128),  and  the  taste  buds  failed  to  abolish  skin  sensitiveness, 
which  Parker  therefore  concludes  must  depend  on  free 
nerve  endings  in  the  skin.  He  distinguishes  three  forms 
of  chemical  sensibility  in  these  lower  vertebrates :  common 
chemical  sensibility,  for  which  free  nerve  endings  are  the 
organ;  taste,  dependent  on  the  taste  buds;  and  smell, 
dependent  on  the  olfactory  nerves,  and  responding  to  much 


Sensory  Discrimination:   The  Chemical  Sense    in 

more  dilute  solutions  than  the  other  two,  thus  being  ca- 
pable of  acting  as  a  distance  sense  (550).  A  number  of 
species  of  fish  have  been  shown  to  possess  smell,  by  demon- 
strating that  they  can  discriminate  between  small  bags 
filled  with  food  and  similar  bags  stuffed  with  inedible 
substances,  and  that  this  discrimination  is  lost  when  the 
olfactory  nerves  are  cut  or  the  nostrils  are  closed  (547, 
142).  Shelf ord  and  his  associates  (673)  have  thrown  light 
on  a  very  interesting  problem  in  animal  behavior,  the 
migrations  of  fish.  It  is  well  known  that  salmon  return 
to  fresh  water  to  spawn,  ascending  rivers,  and  that  other 
fish  perform  migrations  that  are  of  great  economic  impor- 
tance to  the  fishing  industry.  Shelford  has  demonstrated 
that  fish  are  very  sensitive  to  slight  variations  in  the  chemi- 
cal constitution,  the  salinity,  for  instance,  of  the  water  in 
which  they  live,  and  their  responses  to  such  changes  may 
well  account  for  all  their  wanderings. 

Among  amphibians,  the  spotted  newt  seems  to  show  a 
relation  between  smell  and  the  "  common  chemical  sense " 
not  unlike  that  existing  in  fishes.  The  olfactory  nerves 
seem  to  be  required  for  the  discrimination  of  food.  When 
chemicals  are  applied  to  the  body,  the  head  end  is  much  the 
most  sensitive  region,  even  when  the  olfactory  nerves  are 
cut.  Acids  and  alkalies  cause  very  marked  reactions ;  salt 
is  less  effective  and  sugar  not  effective  at  all  (629).  Cole 
(135)  studied  the  time  required  for  the  reflex  withdrawal 
of  the  hind  legs  of  leopard  frogs  when  four  chlorides,  those 
of  ammonium,  potassium,  sodium  and  lithium,  were  applied 
in  solution.  He  found  that  the  speed  of  reaction  corre- 
sponded to  the  order  in  which  these  salts  affect  the  human 
sense  of  taste.  That  a  common  chemical  sense,  and  not 
pain,  was  involved  in  these  skin  reactions  was  indicated 
by  the  fact  that  they  persisted  when  ordinary  pain  reactions, 


ii2  The  Animal  Mind 

to  pricks,  were  abolished  by  cocaine.  Risser  (638)  reports 
that  while  sight  seems  to  be  more  important  than  smell  in 
determining  the  mature  toad's  reactions  to  food,  tadpoles 
failed  to  distinguish  packets  containing  food  when  their 
nostrils  were  plugged.  Immature  Amblystomas,  which  in 
the  normal  condition  react  positively  both  to  motionless 
food  and  to  moving  inedible  objects,  lost  the  first  type  of 
response  when  their  nasal  pits  were  removed,  and  the  sec- 
ond type  when  their  eyes  were  operated  on  (112). 

In  birds  sight  and  hearing  are  so  well  developed  that 
the  chemical  sense  assumes  less  importance.  Birds  seem 
to  have  a  sense  of  taste :  the  chicks  experimented  on  by 
Lloyd  Morgan,  for  example,  showed  disgust  on  picking 
up  bits  of  orange  peel  instead  of  yolk  of  egg  (506,  pp.  40- 
41).  Herring  gulls  make  similar  manifestations  on  being 
fed  salt  fish,  and  take  bread  soaked  in  meat  juice  more 
readily  than  water-soaked  bread  (697).  Raspail  (626) 
thinks  that  birds  abandon  eggs  which  have  been  handled 
because  they  detect  the  fact  by  smell;  that  they  find 
buried  grubs  by  smell,  and  are  guided  by  this  sense  to  con- 
cealed food  and  water.  The  last  statement  he  supports 
by  the  observation  that  their  tracks  lead  straight  to  hidden 
food  on  their  first  visit  to  it,  showing  that  it  was  not  found 
by  accident.  Strong  (696)  made  a  careful  study  of  the 
olfactory  apparatus  in  twenty-seven  of  the  thirty-five 
existing  orders  of  birds.  He  concludes  that  "the  olfactory 
organs  of  birds  are  of  too  great  size  to  be  set  aside  as  non- 
functional," but  that  as  one  passes  from  the  lower  to  the 
higher  orders  of  birds  there  is  a  tendency  towards  retrogres- 
sion in  these  organs.  The  crow  family,  sometimes  con- 
sidered to  be  the  highest  birds,  show  extremely  minute 
smell  organs.  "The  sense  of  smell  has  evidently  been 
disappearing  in  birds  with  the  great  development  of  vision." 


Sensory  Discrimination:   The  Chemical  Sense     113 

The  hypothesis  has  been  put  forward  by  Cyon  (165)  that 
smell  may  somehow  function  in  guiding  the  long  flights  of 
birds.  Watson  (770)  found  that  the  noddy  tern  could 
find  its  way  from  Key  West  to  its  nest  on  the  Tortugas 
with  the  nostrils  tightly  sealed.  Strong,  however,  points 
out  that  this  bird  has  very  small  olfactory  organs,  and 
thinks  it  possible  that  other  birds  may  make  more  use  of 
the  olfactory  sense  in  homing  and  migrations.  The  ful- 
mar, for  instance,  is  a  bird  which  makes  very  long  ocean 
flights,  and  has  an  enormously  developed  olfactory  appara- 
tus. Strong  (696)  made  experiments  with  the  ring  dove 
in  which  he  was  apparently  able  to  establish  some  asso- 
ciation between  the  smell  of  bergamot  in  a  certain  compart- 
ment and  the  choice  of  that  compartment  as  containing 
food. 

When  we  come  to  the  Mammalia,  we  find  in  the  great 
majority  of  types  a  very  high  development  of  qualitative 
discrimination  in  the  sense  of  smell.  Hunters  know  it  to 
be  the  chief  defensive  weapon  of  wild  animals,  and  it  has 
retained  great  keenness  in  many  domesticated  ones,  —  the 
cat,  for  instance,  which  will  be  awakened  from  slumber  in 
the  garret  by  the  odor,  quite  unsuspected  of  human  nostrils, 
of  some  favorite  food  being  prepared  in  the  kitchen,  and  is 
thrown  into  ecstasy  at  a  faint  whiff  of  catnip.  The  dog, 
however,  is  the  hero  of  this  field  of  mental  prowess.  The 
experiments  of  Romanes  on  the  power  of  a  favorite  setter 
to  track  his  scent  are  well  known.  In  one  of  them  he  col- 
lected a  number  of  men,  and  told  them  to  walk  in  Indian 
file,  "each  man  taking  care  to  place  his  feet  in  the  footprints 
of  his  predecessor.  In  this  procession,  numbering  twelve 
in  all,"  Romanes  says,  "I  took  the  lead,  while  the  game- 
keeper brought  up  the  rear.  When  we  had  walked  two 
hundred  yards,  I  turned  to  the  right,  followed  by  five  of 


ii4  The  Animal  Mind 

the  men ;  and  at  the  point  where  I  had  turned  to  the  right, 
the  seventh  man  turned  to  the  left,  followed  by  all  the 
remainder.  The  two  parties  .  .  .  having  walked  in  op- 
posite directions  for  a  considerable  distance,  concealed 
themselves,  and  the  bitch  was  put  upon  the  common  track 
of  the  whole  party  before  the  point  of  divergence.  Fol- 
lowing this  common  track  with  rapidity,  she  at  first  over- 
shot the  point  of  divergence,  but  quickly  recovering  it, 
without  any  hesitation  chose  the  track  which  turned  to  the 
right."  It  had  previously  been  ascertained  that  she  would 
not  follow  the  scent  of  any  other  man  in  the  party  save  her 
master,  and  failing  him,  the  gamekeeper.  "Yet  .  .  .  my 
footprints,"  continued  Romanes,  "in  the  common  track 
were  overlaid  by  eleven  others,  and  in  the  track  to  the  right 
by  five  others.  Moreover,  as  it  was  the  gamekeeper  who 
brought  up  the  rear,  and  as  in  the  absence  of  my  trail  she 
would  always  follow  his,  the  fact  of  his  scent  being,  so  to 
speak,  uppermost  in  the  series,  was  shown  in  no  way  to 
disconcert  the  animal  following  another  familiar  scent 
lowermost  in  the  series"  (644).  Such  behavior  indicates 
not  only  that  the  dog  can  experience  a  variety  of  smell 
qualities,  which  is  also  the  case  with  us  human  beings,  but 
that  it  has  the  power  to  analyze  a  fusion  of  different  odors 
and  attend  exclusively  to  one  component,  a  power  that 
we  lack  almost  entirely.  When  we  experience  two  smell 
stimuli  at  the  same  time,  it  is  but  rarely  that  we  can  detect 
both  of  the  two  qualities  in  the  mixture;  usually  one  of 
them  swamps  the  other,  or  else  a  new  odor  unlike  both 
results.  But  the  dog,  and  probably  many  other  animals, 
can  analyze  a  smell  fusion  as  a  trained  musician  analyzes 
a  chord.  In  this  respect,  if  not  in  the  variety  of  smell 
qualities,  the  olfactory  sense  has  undergone  degeneration 
in  us,  and  so  far  as  we  can  judge,  the  fact  is  due  to  the  habit 


Sensory  Discrimination:   The  Chemical  Sense     115 

of  relying  rather  upon  the  sense  of  sight.  Even  in  the 
case  of  the  monkey,  Kinnaman  reports  that  the  animals 
he  was  testing  with  regard  to  their  power  of  discrimi- 
nating the  size,  shape,  and  color  of  vessels  in  one  of 
which  food  was  placed,  always  looked,  never  smelled, 
for  the  food  (401). 


CHAPTER  VI 

SENSORY  DISCRIMINATION:   HEARING 
§  31.   Hearing  in  lower  invertebrates 

THE  sense  of  hearing,  in  all  air-dwelling  animals,  is  that 
sense  whose  adequate  stimulus  consists  in  air  vibrations; 
for  human  beings  these  vibrations  may  reach  a  frequency  of 
50,000  (single  vibrations)  in  one  second  and  still  produce  an 
auditory  sensation.  But  the  meaning  of  the  term  "hear- 
ing" for  water-dwelling  animals,  and  hence  for  most  of  the 
lowest  forms  of  animal  life,  is  more  difficult  to  determine. 
In  the  Protozoa  it  seems  to  have  no  meaning  at  all;  the 
reactions  of  these  animals  to  water  vibrations  are  indistin- 
guishable from  their  reactions  to  mechanical  stimulation. 
But  in  some  of  the  ccelenterates  the  possibility  of  a  specific 
auditory  sensation  quality  has  been  suggested  by  the  dis- 
covery of  a  peculiar  sense  organ.  While  varying  in  its 
structure  in  different  genera  and  orders  of  ccelenterate 
animals,  this  organ  consists  typically  of  a  small  sac,  filled 
with  fluid  and  containing  one  or  more  mineral  bodies. 
Apparently  these  latter  could  operate  in  connection  with  a 
stimulus  only  when  the  stimulus  was  constituted  by  shaking 
the  animal,  or  in  some  way  disturbing  its  equilibrium. 
They  might  then  serve  as  means  for  the  reception  of  water 
vibrations,  as  the  ear  serves  for  the  reception  of  air  vibra- 
tions ;  they  might,  in  short,  be  primitive  organs  of  hearing. 
Accordingly  the  term  "otocysts"  was  given  to  organs 
of  this  type  wherever  they  were  found  in  the  animal 

116 


Sensory  Discrimination:  Hearing  117 

kingdom,  and  the  mineral  bodies  in  the  otocysts  were 
called  otoliths. 

But  experiments  upon  ccelenterates  have  entirely  failed  to 
show  that  animals  of  this  class  react  to  sounds  (205,  741, 
521).  And  in  some  ccelenterates,  as  well  as  in  higher 
animals  having  the  same  type  of  organ,  the  removal  of  the 
so-called  otocysts  has  been  found  to  involve  disturbance  of 
the  animal's  power  to  keep  its  balance  and  maintain  a 
normal  position.  Hence  Verworn  has  suggested  that  for 
"otocyst"  and  "otolith"  the  terms  "statocyst"  and  "stato- 
lith"  might  appropriately  be  substituted  (741).  In 
jellyfish,  indeed,  even  the  balancing  function  of  the  stato- 
cyst organs  appears  doubtful ;  and  it  is  possible  that  they 
function  in  response  to  shaking  and  jarring  (514,  521).  In 
any  case,  there  is  no  evidence  whatever  of  a  specific  auditory 
sensation  in  the  consciousness,  if  such  exists,  of  ccelenterate 
animals. 

Nor  has  any  reaction  to  sound  been  demonstrated  in  either 
the  flatworms  or  the  annelid  worms ;  their  sensitiveness  to 
vibrations  seems  to  be  an  affair  of  mechanical  stimulation. 
Darwin's  experiments  on  this  point  are  well  known.  The 
earthworms  which  he  observed  were  quite  insensitive  to 
musical  tones,  but  when  the  flower  pots  containing  their 
burrows  were  placed  on  a  piano,  the  worms  retreated  hastily 
as  soon  as  a  note  was  struck  (171).  Most  observers  agree 
that  mollusks  also  react  only  to  mechanical  jars  (e.g.,  190), 
and  that  the  statocyst  organs  found  in  some  mollusks  have 
no  auditory  function.  Bateson,  however,  records  that  a 
certain  lamellibranch,  suspended  by  a  thread  in  a  tank,  re- 
sponded by  shutting  its  shell  when  a  sound  was  produced 
by  rubbing  a  finger  along  the  glass  side  of  the  tank  (25). 
The  echinoderms  are  apparently  insensitive  to  auditory 
stimuli  (617,  641). 


n8  The  Animal  Mind 

§32.  Hearing  in  Crustacea 

In  the  Crustacea  the  function  of  the  statocyst  organs  has 
been  the  subject  of  much  dispute.  They  are  in  this  group  of 
animals  sometimes  closed  sacs  with  statoliths,  sometimes 
open  sacs  containing  grains  of  sand.  Most  commonly  the 
organs  are  situated  in  the  basal  segment  of  the  small 
antennae.  There  is  usually  inside  the  sac  a  projection 
bearing  several  ridges  of  hairs,  graded  in  size,  which  tempt 
to  the  hypothesis  that  they  respond  to  vibrations  of  different 
wave  lengths,  as  the  fibres  of  the  basilar  membrane  of  the 
human  cochlea  are  supposed  by  the  Helmholz  theory 
to  do.  Hensen,  indeed,  placing  under  the  microscope  the 
tail  of  a  small  shrimp,  My  sis,  whose  statocyst  is  situated 
in  that  region,  observed  that  the  long  hairs  of  the  tail 
vibrated  in  response  to  musical  tones,  from  which  he 
infers  that  the  statocyst  hairs  may  do  so1  (294).  In 
1899  he  was  still  inclined  to  believe  that  the  latter  can 
serve  no  other  than  an  auditory  function  (295).  Never- 
theless the  weight  of  authority  is  in  favor  of  regarding  the 
"sac"  in  Crustacea  as  a-  static  rather  than  an  auditory 
organ.  The  only  evidence  of  sound  reaction  in  two  shrimp- 
like  forms,  Palaemon  and  Palaemonetes,  was  a  "flight  reflex" 
given  by  some  individuals  when  sounds  were  produced  very 
near  them  in  the  water ;  and  although  this  response  ceased 
when  the  statocysts  were  destroyed,  the  fact  is  of  little  sig- 
nificance, as  other  reflexes  also  were  abolished  by  the  opera- 
tion (38) .  To  sounds  made  by  tapping  the  wall  of  the  aqua- 
rium Palaemonetes  reacted  by  leaping  away  from  the  wall 
nearest  to  it,  even  though  the  leap  was  made  toward  the 

1  This  observation  is  sometimes  incorrectly  quoted  as  if  the  hairs  con- 
cerned were  actually  the  statocyst  hairs.  C/.,  for  example,  Morgan,  504, 
p.  266. 


Sensory  Discrimination:  Hearing  119 

sound.  When  both  statocysts  were  removed,  the  reactions 
were  still  made,  but  not  so  markedly  nor  at  so  great  a  dis- 
tance from  the  sound.  A  similar  response  to  the  striking 
of  a  partially  submerged  glass  jar  was  seen  in  a  decapod, 
Virbius  zostericola,  which  has  no  statoliths  (616).  Mysis 
has  been  found  to  react  to  sounds  when  the  statocysts  are 
destroyed  (48).  The  fiddler  crab,  which  is  amphibious, 
responds  in  water  to  vibrations  by  retreating  slowly  from 
the  vibrating  walls,  and  does  the  same  when  blinded  and 
deprived  of  its  statocysts,  but  gives  no  reaction  when  the 
antennae  and  antennules  are  removed.  On  land  these 
animals  do  not  respond  to  sounds,  only  to  vibrations  pro- 
duced in  the  earth,  for  instance  by  stamping  (616).  No 
sound  reactions  have  been  found  in  the  crayfish  (40).  In 
short,  such  responses  to  vibrations  as  occur  among  the 
Crustacea  seem  affairs  rather  of  mechanical  than  of  true 
auditory  stimulation ;  nevertheless  Be  the  (48)  and  Hensen 
(295)  are  both  inclined  to  believe,  as  did  Delage,  who  first 
called  attention  to  the  static  function  of  the  statocysts 
(180),  that  they  may  be  auditory  organs  also.  The  "static 
sense"  of  Crustacea  will  be  discussed  later. 


§  33.   Hearing  in  Spiders 

In  spiders  the  same  difficulty  arises,  of  deciding  whether 
the  reactions  to  sound  are  tactile  or  auditory.  There  are  no 
statocysts,  but  the  delicate  hairs  on  the  body  and  legs  of  the 
animal  have  been  held  to  be  auditory  organs.  Dahl,  a 
number  of  years  ago,  found  them  responding  to  the  tones  of 
a  violin  (166,  167),  but  this  test,  which  Hensen  applied  to 
Mysis,  is  of  very  doubtful  significance ;  as  Prentiss  suggests, 
the  hairs  on  the  back  of  the  human  hand  do  the  same  (616). 
When  various  species  of  spiders  were  tested  by  holding 


120  The  Animal  Mind 

tuning  forks  near  them  or  their  webs,  only  the  web-making 
species  gave  any  response.  These  latter  would  not  react 
to  ordinary  noises,  nor  to  the  sound  of  a  small  fork,  but  to 
the  humming  of  a  large  fork  they  responded  always  by 
raising  the  front  legs,  and  sometimes  by  dropping  from  their 
webs  (570).  Two  Texan  species  that  were  experimented 
upon  by  placing  them  in  a  cage  free  from  vibration  gave  no 
response  whatever  to  tuning  forks  of  various  pitches  or  to 
other  sounds  (618).  It  seems,  then,  highly  probable  that 
spiders  are  sensitive  only  to  vibrations  communicated  to 
their  webs,  and  very  likely  these  furnish  tactile  rather  than 
specific  auditory  stimulation.  The  observation  of  Boys 
may  be  quoted:  "On  sounding  an  A  fork,  and  lightly 
touching  with  it  any  leaf  or  other  support  of  the  web  or 
any  portion  of  the  web  itself,  I  found  that  the  spider,  if  at 
the  centre  of  the  web,  rapidly  slews  around  so  as  to  face  the 
direction  of  the  fork,  feeling  with  its  fore  feet  along  which 
radial  thread  the  vibration  travels.  Having  become 
satisfied  on  this  point,  it  next  darts  along  that  thread  till  it 
reaches  either  the  fork  itself  or  a  junction  of  two  or  more 
threads,  the  right  one  of  which  it  instantly  determines  as 
before.  If  the  fork  is  not  removed  when  the  spider  has 
arrived  it  seems  to  have  the  same  charm  as  any  fly,  for  the 
spider  seizes  it,  embraces  it,  and  runs  about  on  the  legs  of 
the  fork  as  often  as  it  is  made  to  sound,  never  seeming  to 
learn  by  experience  that  other  things  may  buzz  besides  its 
natural  food.  If  the  spider  is  not  at  the  centre  of  the  web 
at  the  time  that  the  fork  is  applied,  it  cannot  tell  which  way 
to  go  until  it  has  been  to  the  centre  to  ascertain  which  radial 
thread  is  vibrating."  If,  however,  it  has  followed  the  fork 
to  the  edge  of  the  web,  and  the  fork  is  then  withdrawn  and 
brought  near  again,  the  spider  reaches  out  in  its  direction. 
If  the  spider  is  at  the  centre  of  the  web  and  a  sounding  fork  is 


Sensory  Discrimination:  Hearing  121 

brought  near  without  touching  the  web,  the  spider  does  not 
reach  for  it,  but  drops  down  at  the  end  of  a  thread.  If  the 
fork  touches  the  web  again,  the  spider  climbs  the  thread  and 
finds  the  spot  very  quickly  (100). 

§  34.   Hearing  in  Insects 

The  sense  of  hearing  in  insects  also  is  problematical. 
When  the  insect  makes  a  sound  itself,  which,  as  in  the  case  of 
crickets,  is  connected  with  the  mating  process,  it  would  seem 
a  priori  highly  probable  that  it  can  hear.  Various  struc- 
tures have  been  designated  as  auditory  organs,  the  finely 
branched  antennae  of  mosquitos  and  gnats,  on  the  same 
doubtful  evidence  that  they  have  been  found  to  vibrate 
in  response  to  musical  tones  (479) ;  and  in  the  Orthoptera 
certain  very  peculiar  structures  situated  on  the  front  legs  of 
grasshoppers  and  crickets,  and  in  the  first  segment  of  the 
abdomen  in  locusts.  These  structures  Graber  called  chor- 
dotonal  organs,  and  he  felt  convinced  from  experimental 
tests  that  they  were  auditory.  The  cockroach,  Blatta, 
while  running  about  the  room  will  stop,  he  says,  for  an 
instant  when  the  strings  of  a  violin  are  struck.  A  blinded 
specimen,  hung  by  a  thread,  became  violently  agitated  at  a 
sudden  tone  from  a  violin.  A  water  insect,  Corixa,  al- 
though undisturbed  by  the  water  vibrations  produced  by 
pushing  a  bone  disk  toward  it  in  the  water,  gave  decided 
reactions  when  the  disk  was  connected  with  an  electric  bell. 
Other  water  beetles  were  still  more  sensitive.  That  they 
distinguished  pitch  differences  Graber  thought  probable 
from  the  fact  that  he  observed  reactions  of  different  degrees 
of  violence  to  sounds  of  different  pitch;  and  their  dis- 
crimination of  intensity  changes  he  thought  demonstrated 
by  the  fact  that  if  a  continuous  tone,  sounding  while  a  water 


122  The  Animal  Mind 

beetle  is  swimming  about,  be  made  suddenly  louder,  the 
speed  of  the  insect's  movements  visibly  increases.  It  is 
going  rather  far,  however,  to  pass  from  the  evidence 
that  insects  discriminate  sounds  made  by  their  own 
species  from  other  sounds  to  the  conclusion  that  "they 
like  us  have  the  capacity  to  analyze,  at  least  to  a  certain 
degree,  these  peculiar  clangs  or  noises,  and  to  distinguish 
clearly  from  one  another  the  partial  tones  that  compose 
them"  (264). 

Tower  thought  that  he  had  observed  the  potato  beetle 
reacting  to  the  sound  of  a  tuning  fork  (717).  Will  noted 
responses  from  a  male  beetle  to  the  stridulation  of  a  female 
of  its  species  enclosed  in  a  box  15  cm.  away  (786).  Radl 
made  the  suggestion  that  the  organs  which  Graber 
called  chordotonal  organs,  and  which  contain  a  fibre 
stretched  between  two  points  of  the  integument,  represent 
a  kind  of  transition  between  "Gemeingeftthl"  and  hearing. 
In  support  he  offers  the  following  evidence :  the  fibres  re- 
semble the  tendons  in  which  some  muscles  end,  and  are  very 
likely  developed  from  tendons ;  the  organs  exist  in  insects 
that  have  no  use  for  hearing,  such  as  grubs  shut  up  in  fruits ; 
insects  have  not  been  shown  to  respond  to  pure  tones,  but 
only  to  noises,  such  as  the  cricket's  chirping,  which  for  us 
affect  GemeingefiihL  Further,  there  is  no  evidence  that 
hearing  ever  guides  insects  to  each  other;  in  short,  it  is 
but  a  rudimentary  sense,  and  its  organs  are  those  which 
serve  also  to  register  muscular  activity.  It  is,  in  insects,  a 
"refined  muscular  sense"  (624).  Regen  (630)  demon- 
strated very  prettily  an  apparently  auditory  reaction  in  the 
female  cricket.  He  placed  in  the  centre  of  a  wide  area  on 
the  floor  two  glass  vessels,  one  lined  with  black  paper, 
the  other  transparent.  In  the  opaque  vessel  he  placed  a 
chirping  male;  in  the  transparent  vessel  a  quiet  male. 


Sensory  Discrimination:  Hearing  123 

Normal  females  ran  to  the  vessel  which  contained  the 
chirping  male,  but  ignored  the  other  vessel:  females 
whose  "tympanal  organs"  on  the  forelegs  had  been 
operated  on  did  not  react  to  either  vessel.  That  the 
response  was  not  to  an  odor  liberated  by  the  move- 
ment of  the  male's  wings  in  chirping,  was  shown  by 
removing  the  edges  of  the  wings,  so  that  their  motion, 
while  otherwise  unchanged,  was  noiseless :  the  response  of 
the  females  ceased. 

It  seems  likely  that  the  auditory  sense,  if  it  exists  in 
insects,  would  be  confined  to  those  which  produce  sounds, 
and  its  qualities  limited  within  the  range  of  such  sounds. 
Turner  (731),  however,  finds  that  silkworm  moths,  alighted 
on  hanging  shelves  and  thus  protected  from  jarring,  respond 
by  waving  their  wings  when  an  organ  pipe,  a  pitch  pipe, 
and  various  notes  on  the  Gal  ton  whistle  are  sounded. 
One  species,  which  failed  to  respond,  he  rendered  more 
excitable  by  rough  handling,  and  then  succeeded  in  stimu- 
lating the  sound  reactions.  Several  different  species  of 
Catocalo  moths  were  found  to  respond  to  high  notes  on  the 
Gal  ton  whistle,  either  by  flying  or  by  quivering  their 
wings.  By  touching  the  insect  at  the  moment  when  the 
tone  was  sounded,  thus  giving  it  a  " life  significance"  to  the 
insect,  some  of  the  moths  were  trained  to  react  to  a  lower 
organ  tone  (256  vibrations)  even  when  they  were  not 
touched.  These  moths  are  not  known  to  make  sounds 
(733).  Most  species  of  ants  produce  no  sound  that  the 
human  ear,  even  with  the  aid  of  a  microphone  (441),  can 
detect,  although  certain  East  Indian  species  are  reported 
to  make  a  loud  hissing  noise  when  disturbed  (760),  and 
some  American  species  are  said  to  chirp  (202,  782). 
Ch.  Janet  maintains  that  ants  of  the  Myrmicidae  make 
a  stridulating  noise  (357,  358).  The  weight  of  evidence 


124  The  Animal  Mind 

is  also  against  the  existence  of  sound  reactions  in  ants; 
careful  experiments  by  Fielde  and  Parker  on  a  number  of 
species  led  to  the  conclusion  that  the  only  vibrations 
responded  to  were  those  which  were  communicated 
through  the  solid  on  which  the  ants  stood,  and  received 
through  the  legs  (226).  It  is  probable  that  the  obser- 
vers who  have  come  to  opposite  conclusions  have  not  in 
every  case  been  careful  to  exclude  the  possibility  of  such 
vibration  of  the  substratum.  Wasmann,  for  instance, 
thinks  he  has  seen  reactions  to  sound ;  he  noted  that  ants 
in  an  artificial  nest  raised  their  antennae  and  lifted  the  fore 
part  of  their  bodies  when  he  scratched  with  a  needle  on  some 
sealing  wax  with  which  the  nest  had  been  mended  (759). 
He  also  quotes  Forel's  account  (230)  of  a  species  which 
makes  an  " alarm  signal"  by  striking  the  ground  with  its 
abdomen :  this,  remarks  Wasmann  naively,  must  be  per- 
ceived by  the  ants,  "otherwise  it  would  not  be  an  alarm 
signal"!  (760).  If  perceived,  it  may  of  course  be  as  a 
tactile  rather  than  an  auditory  sensation.  Weld  has  ob- 
served reactions  to  the  sound  of  whistles  and  tuning  forks 
in  several  species  of  ants,  and  even  concludes  that  they 
perceive  the  direction  from  which  sounds  come ;  but  since, 
of  the  four  observations  upon  which  this  latter  opinion  is 
based,  two  were  cases  where  the  ants  hurried  toward  the 
sound  and  the  others  cases  where  they  backed  away  from 
it,  the  possibility  of  mere  coincidence  seems  not  to  be 
excluded  (776). 

As  regards  the  auditory  sense  in  bees,  there  is  again  a 
difference  of  opinion.  They  do,  of  course,  make  sounds, 
and  sounds  of  different  quality,  under  different  conditions. 
Yet  Lubbock  entirely  failed  to  get  bees  to  respond  to  any 
kind  of  sounds  artificially  produced  (441),  while  Bethe  urges 
that  the  sounds  produced  by  bees  are  involuntary,  like  the 


Sensory  Discrimination:  Hearing  125 

sounds  of  our  own  breathing  and  heart-beats,  and  that  there 
is  no  more  evidence  that  bees  can  hear  them  than  that  we 
can  hear  these  sounds  in  our  own  case  (52).  Forel  is 
positive  that  insects  in  general  cannot  hear  (231).  Von 
Buttel-Reepen,  on  the  other  hand,  who  knows  bees  thor- 
oughly, thinks  that  the  sense  of  hearing  plays  a  considerable 
part  in  their  life.  He  believes  that  the  disturbance  pro- 
duced by  the  loss  of  a  queen  is  communicated  to  the  whole 
hive  by  the  peculiar  wailing  noise  made  by  some  members 
and  instinctively  imitated  by  the  others,  and  that  this 
disturbance  is  calmed  by  a  similar  dissemination  of  the 
" happy  humming"  produced  on  her  restoration  —  hearing 
playing  a  more  important  part  than  smell.  The  starting 
of  a  swarm,  he  thinks,  is  also  largely  a  matter  of  sound 
communication.  The  process  begins  by  the  coming  out 
of  certain  bees  which  push  in  among  the  bees  hanging  at  the 
entrance  of  the  hive  and  stir  them  up  to  swarming  by  mak- 
ing sounds.  The  " swarm-tone"  is  peculiar  and  often 
disturbs  the  inhabitants  of  neighboring  hives  that  are  not 
ready  to  swarm.  Also,  a  swarm  can  be  guided  to  a  new 
dwelling  if  a  few  bees  are  taken  there ;  they  call  the  others 
by  loud  humming.  If  during  this  process  the  new  hive  is 
moved,  the  bees  will  go  on  for  a  few  moments]  in  the 
direction  in  which  they  started,  then  slowly  turn,  guided  by 
the  tone.  A  few  may  keep  on  in  the  original  direction. 
We  may  look  with  suspicion,  however,  upon  von  Buttel- 
Reepen's  suggestion  that  these  latter,  having  passed  be- 
yond hearing  of  the  call,  are  guided  by  the  recollection 
of  the  tone  they  heard  at  first!  He  refers  also  to  the 
shrill  noise  made  by  the  young  queens  ready  to  swarm, 
and  to  the  peculiar  uneasiness  produced  when  a  strange 
queen  is  being  attacked,  and  resulting,  he  thinks,  from 
her  "cries  of  pain"  (115). 


126  The  Animal  Mind 

§  35.   Hearing  in  Fishes 

Throughout  the  vertebrate  animals  there  exist  structures 
bearing  analogy  to  our  own  ears,  whose  function  might 
therefore  be  supposed  to  be  auditory.  But  in  the  lowest 
vertebrates  the  only  structures  of  the  human  ear  represented 
are  the  semicircular  canals,  and  these  suggest  a  static  rather 
than  an  auditory  organ.  The  cyclos tomes,  eel-like  and 
semiparasitic  forms  classed  below  the  true  fishes,  have  a 
pair  of  sacs  one  on  either  side  of  the  head,  containing 
mineral  bodies,  and  each  leading  into  one  or  two  semi- 
circular canals.  In  the  true  fishes  the  sac  has  two  chambers, 
marked  off  from  each  other  by  a  constriction.  Three  semi- 
circular canals  open  from  the  foremost  chamber,  two  lying 
in  the  vertical  plane,  and  one  in  the  horizontal  plane. 
The  chambers  contain  "statoliths"  and  fluid. 

That  the  semicircular  canals  in  fishes  have  a  static 1  func- 
tion has  been  shown  by  experiments  to  be  described  later. 
Is  the  fish  ear  also  an  organ  of  hearing  ?  Again  authorities 
disagree,  and  it  is  probable  that  species  differ.  Kreidl  got 
no  response  from  goldfish  when  vibrating  rods  were  placed 
either  in  the  water  or  in  the  air  near  the  water.  Only 
when  the  fish  were  made  more  sensitive  by  strychnin  did 
they  react,  and  only  to  noise,  not  to  tone.  They  reacted 
quite  as  well,  moreover,  when  the  ears  were  removed; 
whence  it  was  concluded  that  their  sensitiveness  to  noise 
resided  in  the  skin  (408,  409).  A  similar  negative  con- 
clusion regarding  auditory  sensation  has  been  reached  by 
F.  S.  Lee  (416),  by  O.  Korner  as  a  result  of  experiments 
on  twenty-five  species  (404),  and  by  Marage  (460  a),  using 

JThe  word  "static"  is  here  used  to  mean  "relating  to  equilibrium" 
in  general,  not  to  static  equilibrium  as  distinguished  from  dynamic 
equilibrium. 


Sensory  Discrimination:  Hearing  127 

vowel  sounds  sung  on  notes  ranging  from  C%  to  Ge,  trans- 
mitted through  rubber  tubes,  the  tests  being  made  on  eight 
species.  On  the  other  hand,  Bigelow  found  that  the  gold- 
fish on  which  he  experimented  were  sensitive  in  their 
normal  condition,  but  insensitive  when  the  auditory  nerves 
were  cut,  and  thinks  that  Kreidl's  operation  did  not  remove 
the  whole  of  the  fish's  ear  (54) .  Triplett  thought  both  perch 
and  goldfish  were  excited  by  the  sound  of  whistling,  which 
usually  preceded  their  being  fed  (720).  Parker  tested  the 
killifish,  a  species  of  minnow,  using  the  sustained  slow  vibra- 
tions (40  complete  swings  per  second)  of  a  bass  viol  string 
placed  on  one  side  of  the  aquarium  as  a  sounding  board.  The 
fish  cage  was  suspended  in  the  aquafium  from  an  indepen- 
dent support.  Normal  fish  responded  to  the  vibrations, 
usually  by  movements  of  the  fin,  96  per  cent,  of  the  time. 
Fish  in  which  the  nerves  to  the  ears  had  been  cut  responded 
in  1 8  per  cent,  of  the  tests ;  those  in  which  the  skin  had  been 
made  insensitive,  but  the  ears  left,  in  94  per  cent.  Since 
causing  the  string  to  vibrate  jarred  the  whole  aquarium 
somewhat,  these  experiments  were  checked  by  others  where 
the  stimulus  was  produced  by  placing  the  stem  of  a  vibrat- 
ing tuning  fork  against  the  sounding  board.  The  results 
were  the  same  as  in  the  first  set  of  tests.  Parker  concludes 
that  the  ears  of  the  minnow  are  certainly  organs  for  the 
reception  of  sound ;  but  as  he  obtained  no  such  reactions 
from  dogfish,  he  is  inclined  to  think  that  different  species 
vary  (535?  536).  In  later  experiments  (544)  on  the  dogfish, 
Parker  finds  that  individuals  with  the  "  auditory  "  or  eighth 
nerve  cut  show  diminished  sensitiveness  to  the  blow  of  a 
pendulum,  the  force  of  whose  impact  on  the  walls  of  the 
aquarium  could  be  measured,  while  cutting  the  optic  nerve 
or  cocainizing  the  skin  has  no  effect  on  the  responses  to 
these  stimuli:  his  conclusion  is  that  the  reactions  are 


128  The  Animal  Mind 

auditory.  In  the  squeteague  (542),  he  infers  from  the 
results  of  operation  that  one  part  of  the  ear,  the  utriculus, 
functions  in  the  maintenance  of  equilibrium,  while  the  other 
part,  the  sacculus,  is  the  organ  of  hearing.  The  otoliths, 
or  statoliths,  in  the  ears  of  the  squeteague  and  dogfish 
Parker  thinks  have  actually  an  auditory  function,  contrary 
to  what  is  known  of  their  use  in  invertebrate  animals; 
when  they  were  removed  from  the  ear  of  the  dogfish,  he 
reports,  there  was  no  disturbance  of  equilibrium,  but  a 
reduction  in  the  reaction  to  blows  on  the  aquarium  wall, 
and  when  the  large  otolith  in  the  sacculus  of  the  squeteague 
was  pinned  down,  a  similar  result  was  obtained.  Most 
sounds  made  in  the  air  are  extremely  faint  under  water,  but 
to  so,unds  really  propagated  through  water,  Parker  thinks 
many  fish  are  sensitive.  Certain  sounds  may  actually 
attract  them :  the  squeteague,  for  instance,  itself  makes 
sounds  which  may  serve  to  bring  the  sexes  together. 
Tests  by  Zenneck  on  Leuciscus  mtilus,  L.  dobula,  and 
Alburnus  lucidus  also  led  to  the  conviction  that  these  fish, 
at  least,  could  hear.  A  bell  was  struck  by  electricity  under 
water,  and  occasionally  a  piece  of  leather  was  placed  upon  it 
at  the  point  where  the  clapper  struck.  In  the  latter  case 
the  mechanical  vibrations  produced  were,  it  was  held,  the 
same  as  those  occasioned  by  the  actual  ringing  of  the 
bell,  but  the  sound  vibrations  were  destroyed.  The  fish 
reacted  by  swimming  instantly  away  from  the  neighbor- 
hood of  the  bell  when  it  was  rung,  but  not  when  the 
leather  was  used;  hence,  apparently,  they  reacted  to 
sound  (840).  These  experiments,  however,  have  been 
repeated  on  trout  and  eels  by  Bernoulli  (43)  with  nega- 
tive results. 

Widely  distributed  among  fishes  is  a  curious  set  of  struc- 
tures known  as  the  lateral-line  canals.    Along  each  side  of 


Sensory  Discrimination:  Hearing  129 

the  fish,  extending  from  head  to  tail,  there  is  a  row  of 
pores  opening  into  a  long  canal,  which  at  the  head  divides 
into  three  branches,  one  going  upward  above  the  eye,  a 
second  below  the  eye,  and  a  third  down  toward  the  lower 
jaw.  The  functions  of  these  canals  have  given  rise  to 
much  discussion  among  zoologists,  an  exhaustive  history 
of  which  will  be  found  in  Parker's  monograph  entitled 
"The  Function  of  the  Lateral-line  Organs  in  Fishes." 
Parker  first  proved  experimentally  that  the  canals  played 
no  part  in  responses  to  the  following  stimuli :  light,  heat, 
salinity  of  the  water,  food,  oxygen  dissolved  in  the  water, 
carbon  dioxide,  foulness  of  the  water,  hydrostatic  pressure, 
steady  currents  flowing  through  the  water,  and  sound. 
When,  however,  the  water  in  the  aquarium  was  made  to 
vibrate  slowly,  about  six  times  per  second,  the  fish  made 
certain  characteristic  reactions,  differing  somewhat  for 
the  four  or  five  species  observed,  but  always  failing  to 
appear  when  the  lateral-line  nerve  was  cut.  Parker 
concludes  that  "the  stimulus  for  the  lateral-line  organs  (a 
water  vibration  of  low  frequency)  is  a  physical  stimulus 
intermediate  in  character  between  that  effective  for  the 
skin  (deforming  pressure  of  solids,  currents,  etc.)  and  that 
for  the  ear  (vibrations  of  high  frequency),  and  indicates 
that  these  organs  hold  an  intermediate  place  between  the 
two  sets  of  sense  organs  named"  (539).  The  ear  is  thus 
regarded  as  actually  derived  from  the  lateral-line  canal, 
as  this  in  turn  was  derived  from  the  skin.  We  may  suppose 
that  at  least  three  different  sensation  qualities  result  from 
stimulation  of  the  skin,  the  canals,  and  the  ear,  where 
hearing  can  be  shown  to  exist. 

Hofer  (326  a)  criticizes  these  experiments  on  the  ground 
that  when  Parker  cut  the  lateral-line  nerves  he  also  de- 
stroyed the  nerves  supplying  the  skin  of  the  head,  a  par  tic- 


130  The  Animal  Mind 

ularly  sensitive  region  to  touch  stimuli.  It  is,  according 
to  Hofer,  the  skin  nerves  that  are  affected  by  the  slow 
vibrations  which  Parker  thought  to  be  the  proper  stimulus 
for  the  lateral-line  organs,  and  in  certain  cases  he  demon- 
strated that  such  stimuli  were  responded  to  when  the  lateral- 
line  organs  had  been  destroyed.  The  true  function  of  the 
lateral-line  organs  Hofer  finds  to  be  that  of  response  to 
streaming  movements  in  the  water.  A  skin  sensitive- 
ness to  currents  would  be  of  the  greatest  practical  value  in 
guiding  the  fish's  migrations. 

§  36.   Hearing  in  Amphibia 

Emergence  from  the  water,  on  the  part  of  adult  Amphibia, 
is  accompanied  by  disappearance  of  the  lateral-line  canals, 
and  consequently  of  whatever  sensations  these  mediate.  In 
the  frog,  the  ear  has  a  tympanic  membrane  lying  at  the  sur- 
face of  the  head.  A  single  bone,  the  columella,  with  one 
end  against  this  membrane,  lies  across  the  middle  ear.  The 
internal  ear  is  not  essentially  different  in  structure  from  that 
of  the  fish;  there  is  no  cochlea.  Yerkes  has  made  an 
interesting  study  of  the  reaction  of  frogs  to  sound.  He 
found  that  they  occasionally  "  straightened  up  and  raised  the 
head  as  if  listening"  when  other  frogs  croaked  or  made  a 
splash  by  jumping  into  the  water.  To  no  other  sound  did 
he  get  any  apparent  response,  nor  was  it  possible  to  make 
frogs  in  their  native  habitat  jump  or  show  any  uneasiness 
by  producing  any  sort  of  noise,  so  long  as  the  experimenter 
remained  invisible.  "Apparently,"  Yerkes  says,  "they 
depend  almost  entirely  upon  vision  for  the  avoidance  of 
dangers."  It  is  of  course  highly  improbable  that  an  organ 
should  be  adapted  only  to  the  reception  of  the  croaking  of 
other  frogs  and  the  splash  of  water,  and  not  to  noises  made 


Sensory  Discrimination:  Hearing  131 

in  imitation  of  these ;  and  Yerkes  suggests  that  the  frogs 
may  hear  many  sounds  to  which  they  respond  by  inhibiting 
movement  as  a  measure  of  safety.  This  view  is  confirmed 
by  the  results  of  experiments  where  the  breathing  move- 
ments of  the  frog's  throat  were  registered  by  means  of  a 
lever  resting  against  it  and  recording  on  smoked  paper. 
Evidence  from  change  of  the  breathing  rate  was  obtained 
of  the  hearing  of  sounds  ranging  from  fifty  to  one  thousand 
single  vibrations  a  second  (807).  Later,  it  was  shown  that 
sounds,  although  they  did  not,  when  given  alone,  cause  the 
frogs  to  react,  modified  the  responses  to  other  stimuli, 
reinforcing  or  inhibiting  them  according  to  the  interval 
between  the  sound  and  the  other  stimulus.  This  effect  was 
noticed  both  when  the  frogs  were  in  the  air  and  when  they 
were  under  water.  It  was  more  marked  in  the  spring  (the 
mating  season)  than  in  the  winter.  That  it  concerned  the 
special  auditory  sense-apparatus,  and  hence  may  have  been 
accompanied  by  true  auditory  sensations,  was  shown  by  the 
fact  that  it  disappeared  when  the  auditory  nerves  were  cut. 
Sounds  ranging  from  fifty  to  ten  thousand  single  vibrations 
a  second  were  effective  (817,  815).  This,  of  course,  does  not 
mean  that  the  frog  perceives  such  sounds  as  differing  in 
pitch. 

§  37.   Hearing  in  Higher  Vertebrates 

The  reptilian  ear  does  not  differ  markedly  from  that  of 
amphibians.  The  writer  knows  of  no  experiments  upon 
the  sense  of  hearing  in  reptiles.  The  cochlea,  the  organ 
of  hearing  in  mammals,  is  still  imperfectly  developed  in 
birds.  But  if  we  grant  that  animals  which  produce 
sounds  are  capable  of  hearing  them,  some  birds  at 
least  must  be  able  to  make  pitch  discriminations  of  wide 
range  and  great  acuteness.  The  powers  of  imitation  so 


132  The  Animal  Mind 

often  evidenced  in  bird  song  are  proof  that  this  is  the  case.1 
Craig  (157, 158)  has  carefully  observed  the  social  significance 
of  a  great  variety  of  sounds  made  by  pigeons,  but  gets  little 
evidence  that  these  birds  learn  new  sounds  by  imitation. 
Extremely  significant  are  Hunter's  (353,  354)  experi- 
ments on  the  hearing  ability  of  the  white  rat.  Their 
net  result  is  that  these  animals  can  hear  only  noises, 
not  tones.  None  of  the  rats  he  tested  was  able  to  hear 
a  tuning  fork  tone ;  the  evidence  is  that  they  were  unable, 
under  the  stimulus  of  both  punishment  and  reward,  to 
learn  to  turn  to  the  right  when  the  tone  was  sounded 
and  to  the  left  when  it  was  not  sounded.  They  could 
perfectly  well  acquire  such  a  habit  when  the  noise  of  clap- 
ping the  hands  was  substituted  for  the  tone.  They  could 
not  form  the  habit  when  two  forks  of  different  pitch  were 
sounded  together  as  a  signal  to  turn  to  the  right.  They 
acquired  the  habit  of  making  the  proper  response  when  the 
tones  of  a  whistle  were  substituted  for  the  fork  tones,  but 
it  was  clear  that  they  were  really  responding  to  the  noise 
of  the  rush  of  air  through  the  whistle,  for  they  would 
react  equally  well  when  this  noise  was  substituted  for  the 
actual  blowing  of  the  whistle.  Moreover,  they  broke 
down  in  their  choices  when  the  whistle  was  sounded  in 
another  room,  although  they  were  not  disturbed  by  the 
mere  diminution  of  intensity  in  the  sound  of  the  whistle 
sounded  near  at  hand;  the  natural  inference  is  that  re- 
moving the  whistle  to  a  distance  made  the  noise  accom- 
panying its  tone  inaudible.  In  short,  there  seemed  to  be, 
for  these  rats,  no  difference  between  the  sound  of  a  pure 
tone  and  entire  silence.  Confirmatory  results  appear  in 

1  Interesting  evidence  of  this  power  in  a  bird  which  might  not  have  been 
supposed  to  possess  it  was  obtained  by  Conradi,  who  found  that  English 
sparrows  reared  by  canaries  acquired  recognizable  bits  of  the  canary  song 
(141)- 


Sensory  Discrimination:  Hearing  133 

Barber's  (19)  experiments  on  the  white  rat's  ability  to 
localize  sounds :  noises,  such  as  those  made  by  tapping  on 
wood,  were  localized  within  an  average  limit  of  error  of 
from  two  to  four  inches,  but  tuning-fork  and  organ-pipe 
tones  were  wholly  ignored. 

The  experiments  of  Johnson  (384)  on  the  dog's  ability  to 
discriminate  tones  and  noises  gave  results  very  similar 
to  these  of  Hunter  on  the  rat,  and  furnish  an  illuminating 
commentary  on  certain  difficulties  in  experimentation  on 
animals.  Zeliony  (839),  working  by  the  salivary  reflex 
method  described  on  page  57,  had  reached  the  conclusion 
that  the  dog  can  discriminate  between  tones  whose  pitch 
differs  by  only  a  quarter  of  a  tone.  Kalischer  (388),  whose 
interest  lay  in  testing  the  work  of  Munk1  on  the  localization 
of  the  central  terminations  of  the  sensory  pathways  for 
tone  in  the  temporal  region  of  the  cortex,  succeeded  in 
training  dogs,  with  and  without  temporal  lobes,  to  snap 
for  food  when  one  tone  was  sounded  and  inhibit  reaction 
when  a  tone  of  considerably  different  pitch  was  given. 
Rothmann  (646  a)  and  Swift  (699)  also  observed  discrimina- 
tion of  tones  in  the  dog,  and  Kalischer  (388)  claims  for  the 
dog  memory  of  absolute  pitch.  But  the  experiments  of  all 
these  investigators,  including  those  who  used  Pawlow's 
method,  suffer  from  the  fatal  defect  that  the  experimenter 
was  in  the  room  with  the  animal  tested,  and  hence  might 
have  presented  other  clews,  by  making  slight  involuntary 
movements,  which  could  act  instead  of  the  tones  to  guide 
the  animal's  choices.  How  serious  this  objection  is  appears 
from  Johnson's  own  results.  His  dog  subjects  all  learned  to 
discriminate  between  a  tone  of  256  double  vibrations  and 
one  of  384  double  vibrations,  an  interval  of  a  fifth,  whether 
the  tones  were  sounded  on  tuning  forks  or  on  a  wind  appara- 

1  Munk,  H.,  1890.     Ueber  die  Funktion  der  Grosshirnrinde.    Berlin. 


134  The  Animal  Mind 

tus  (the  Stern  tone  variator) ;  and  to  discriminate  between 
a  chord  containing  one  of  these  tones  and  a  chord  containing 
the  other  tone.  This  was  when  the  experimenter  remained 
in  the  room.  Experiments  by  a  much  more  accurate 
apparatus,  where  electrically  driven  forks  were  sounded 
from  another  room,  while  the  experimenter  observed  the  dog 
also  from  an  adjoining  room,  the  tones  being  sounded  with 
exactly  the  same  duration,  gave  precisely  opposite  results : 
the  dogs  could  not  discriminate.  (The  test  of  discrimina- 
tion was  learning  to  turn  to  the  right  when  one  tone  was 
sounded  and  to  the  left  when  the  other  tone  was  sounded.) 
Moreover,  they  could  not  even  learn  to  take  one  turning 
when  a  tone  was  given  and  the  other  when  no  tone  at  all 
was  given;  apparently  if  they  heard  the  tone  at  all  they 
paid  no  attention  to  it.  On  the  other  hand,  the  noises  of 
two  electric  buzzers,  of  different  intensity,  pitch,  and  timbre, 
were  readily  discriminated  and  localized  by  the  dogs. 

The  same  objection,  that  secondary  clews  derived  from 
the  presence  of  the  operator  may  account  for  the  seeming 
discrimination  of  sounds,  applies  to  the  work  of  Shepherd 
on  cats  (677,  678)  and  raccoons  (676).  The  apparent  fact 
that  certain  mammals  are  deaf  to  tones,  while  perfectly 
able  to  hear  noises  may,  as  Johnson  suggests,  be  connected 
with  the  fact  that  even  human  beings  cannot  localize  pure 
tones  with  any  accuracy :  a  sound  stimulus,  to  have 
practical  significance,  must  be  capable  of  being  localized. 
Zeliony  (839)  trained  a  cat  to  come  from  one  room  into 
another  when  a  C'  whistle  was  blown,  and  thought  he  had 
evidence  of  the  cat's  ability  to  distinguish  the  sound  of 
this  whistle  from  that  of  others  differing  not  more  than 
a  half-tone;  but  the  difference  reacted  to  may  have  been 
in  the  accompanying  noises.  Hahn  (282)  finds  the  bat 
very  sensitive  to  high-pitched  sounds,  but  not  to  low  ones. 


CHAPTER  VII 

SENSORY  DISCRIMINATION:  VISION 
§  38.   Change  of  Light  Intensity  as  a  Stimulus 

IN  this  chapter  we  shall  omit  all  references  to  the  func- 
tion of  vision  as  a  spatial  sense,  that  is,  as  giving  rise  to 
perceptions  of  form,  size,  distances,  and  direction. 

It  appears  that  light  may  act  upon  living  beings  either 
as  a  continuous  or  as  an  interrupted  stimulus.  That  is, 
light  maintained  steadily  at  a  constant  intensity  produces 
responses  in  organisms,  and  there  are  also  reactions  when 
the  intensity  of  the  light  is  suddenly  altered,  in  the  direc- 
tion either  of  brightening  or  of  darkening.  Most  physical 
forces  act  as  stimuli  only  when  they  change  in  some  way : 
an  unchanging  environment  fails  to  call  forth  response. 
We  may  briefly  survey  the  facts  which  point  to  the  exist- 
ence in  animals  of  reactions  to  changes  in  light  intensity. 

Among  the  Protozoa,  Amoeba,  when  subjected  to  sud- 
den changes  in  light  intensity,  checks  its  movement  at  the 
point  where  the  light  falls.  This  is  just  what  happens  when 
a  mechanical  stimulus  is  applied,  and  offers  no  evidence 
of  a  specific  light  sensation  (378).  Similarly,  various 
ciliate  and  flagellate  Protozoa  give  their  ordinary  negative 
or  avoiding  reaction  to  changes  in  light  intensity ;  some  of 
them  make  it  on  passing  from  a  region  of  less  to  one  of 
greater  illumination,  and  thus  "seek"  the  darker  regions, 
while  others  give  it  when  undergoing  a  change  in  the  re- 
verse direction,  and  thus  tend  to  remain  in  lighter  regions. 

135 


136  The  Animal  Mind 

But  if  nothing  distinguishes  the  negative  reaction  to 
photic  stimuli  from  the  negative  reaction  to  any  other 
stimulus,  then  nothing  shows  the  existence  of  a  sensation 
quality  peculiar  to  the  effect  of  light  —  unless  a  special 
receptive  apparatus  can  be  demonstrated.  In  a  flagellate 
Protozoon  called  Euglena,  a  pigment  spot  exists  near  the 
anterior  end.  Now  although  pigment  apparently  is  not, 
as  Hesse  (323)  has  emphasized,  a  necessary  constituent  of 
visual  organs,  yet  its  occurrence  always  suggests  some  re- 
lation to  light,  as  it  is  essentially  a  kind  of  matter  having 
the  property  of  absorbing  light.  Euglena  gives  the  nega- 
tive reaction  on  entering  a  shadow.  Is  its  pigment  spot 
really  an  "eye  spot"  and  concerned  in  this  response?  Ap- 
parently the  reaction  occurs  before  the  pigment  spot  has 
entered  the  shadow,  and  as  soon  as  the  transparent  tip 
lying  in  front  of  the  pigment  spot  has  been  pushed  into  the 
shaded  region  (204).  It  is  uncertain,  then,  what  the 
function  of  the  pigment  spot  is.  But  in  another  organism, 
which  is  structurally  intermediate  between  the  single- 
celled  and  the  many-celled  forms,  pigment  spots  do  play  a 
role  in  light  reactions.  This  organism  is  called  Volvox, 
and  it  is  really  a  colony  of  globular  flagellates,  each  with  its 
flagellum  turned  outward,  and  each  with  an  "eye  spot." 
Very  weak  light  has  no  effect  on  the  movements  of  Volvox ; 
moderate  light  causes  movement  toward  the  source  of  light, 
and  very  strong  light  causes  movement  away  from  the 
source  (332).  Accurate  observation  of  these  movements 
indicates  that  the  eye  spots  are  essential  to  them ;  each  in- 
dividual responds  to  a  change  of  illumination  of  its  eye 
spot  (464).  This  much  evidence,  then,  we  have  that  if 
Volvox  possesses  consciousness,  changes  of  light  intensity 
produce  in  it  a  specific  sensation. 

Among  ccelenterates,  response  to   changes  of  light  in- 


Sensory  Discrimination:   Vision  137 

tensity  is  found,  although  in  the  hydroid  colonies  of  Tu- 
bularia  it  appears  to  be  wholly  lacking  (564).  Many 
sea-anemones  are  wholly  unaffected  by  light  stimulation, 
Sagartia  luciae  and  Metridium,  for  example  (286).  Many 
others  have  been  observed  to  contract  when  the  light 
intensity  is  increased  (266,  374,  521).  Eloactis  producta 
expands  its  tentacles  only  in  light  of  low  intensity,  taking 
about  fifteen  minutes  to  do  so  when  covered  with  a  hood, 
and  retracting  in  five  minutes  when  the  light  is  restored. 
This  retraction  is  decidedly  slower  than  that  produced 
by  mechanical  stimulation  (286) ;  thus  we  have  some 
evidence  that  it  is  accompanied  by  a  specific  sensation  qual- 
ity. That  the  responses  to  light  are  more  marked  in  ani- 
mals which  have  been  living  in  comparative  darkness  than 
in  those  taken  from  illuminated  spots,  has  been  shown 
both  for  sea-anemones  and  for  Hydra  (228). 

Many  Medusae  or  jellyfish  also  react  to  light  more  slowly 
than  to  other  forms  of  stimulation.  It  is  true  that  on  Sarsia, 
a  form  tested  by  Romanes  many  years  ago,  light  seemed  to 
act  as  quickly  as  any  other  stimulus.  If  a  flash  of  light 
were  allowed  to  fall  on  the  animal  while  it  was  moving 
about,  " prolonged  swimming  movements"  ensued;  if  it 
was  at  rest,  it  gave  only  a  single  contraction  —  another  in- 
stance of  the  effect  of  physiological  condition  upon  reac- 
tion. Sudden  darkening  produced  no  reaction,  whence 
Romanes  concluded  that  "it  is  the  light  per  se  and  not  the 
sudden  nature  of  the  transition  from  darkness  to  light 
which  in  the  former  experiment  acted  as  the  stimulus." 
There  are,  however,  as  we  shall  see,  other  animals  in  which 
an  increase  of  illumination  brings  about  response  where  a 
decrease  fails,  and  vice  versa.  When  a  beam  of  light  was 
thrown  into  a  bell-jar  containing  many  Sarsiae  and  placed 
in  a  dark  room,  "they  crowded  into  the  path  of  the  beam 


138  The  Animal  Mind 

and  were  most  numerous  at  that  side  of  the  jar  which  was 
nearest  the  light."  " There  can  thus/'  concludes  Romanes, 
"be  no  doubt  about  Sarsia  possessing  a  visual  sense" 
(641,  p.  41).  But  as  these  reactions  are  not  differentiated 
in  any  way,  they  cannot  be  taken  as  evidence  of  a  specific 
sense,  unless  indeed  they  depend  on  a  specialized  sensory 
structure.  This  latter  Romanes  found  to  be  the  case; 
Sarsia  has  pigment  spots  on  the  margin  of  its  bell,  and  its 
response  to  light  ceased  when  these  were  destroyed.  Tiar- 
opsis,  another  jellyfish  studied  by  the  same  observer, 
gave  further  evidence  of  "a  visual  sense"  in  the  fact  that 
it  responded  to  light  more  slowly  than  to  mechanical 
stimulation.  In  Gonionemus,  both  difference  in  reaction 
time  and  dependence  of  response  on  a  special  organ  indi- 
cate that  light  may  produce  a  specific  sensation,  always 
granting  the  presence  of  consciousness.  Yerkes  found  that 
this  jellyfish,  unlike  Sarsia,  reacts  in  the  same  manner  in 
passing  either  from  sunlight  to  shadow  or  the  reverse.  In 
both  cases  it  stops  swimming  and  sinks  to  the  bottom.  A 
sudden  change  of  illumination,  therefore,  checks  its  activity. 
On  the  other  hand,  if  when  the  light  falls  upon  it  the  ani- 
mal is  at  rest,  it  becomes  active  again;  but  sudden  de- 
crease of  illumination  has  no  effect  upon  the  resting  animal. 
The  inhibitory  effect  of  strong  light  falling  upon  the  jelly- 
fish while  in  motion  Yerkes  explains  as  a  special  adapta- 
tion. For  one  case  of  such  increase  of  illumination  occurs 
when  the  animal  swims,  bell  upward,  to  the  surface  on 
being  disturbed ;  the  light  of  the  surface  is  of  course  nor- 
mally stronger  than  that  in  the  lower  regions.  The  in- 
hibition of  activity  resulting  causes  the  animal,  after 
turning  over,  to  sink  slowly,  bell  downward,  with  expanded 
tentacles.  This  is  a  position  that  gives  it  a  better  chance 
of  catching  food  and  carrying  it  to  the  lips  than  is  offered 


Sensory  Discrimination:   Vision  139 

by  the  right-side-up  posture,  where  food  would  have  to  be 
carried  downward  against  the  upward  current  occasioned 
by  the  sinking  of  the  animal.  Light  is  not  the  only  factor 
in  producing  the  inversion  at  the  surface,  however,  for  it 
will  occur  in  darkness.  When  swimming,  Gonionemus 
moves  toward  the  light  if  the  latter  is  fairly  intense,  but 
comes  to  rest  in  the  shaded  portions  of  the  vessel  con- 
taining it.  The  reaction  time  to  light  is  much  slower  than 
that  to  other  stimuli,  but  the  animal  responds  most  promptly 
when  certain  pigmented  bodies  at  the  base  of  the  tentacles 
are  exposed  to  the  stimulus.  If  the  margin  of  the  bell  con- 
taining these  bodies  is  cut  off,  no  reaction  to  light  can  be 
obtained  (802,  809,  825).  A  great  variety  of  structures 
apparently  sensory  in  function  is  found  on  the  bell  margin 
of  different  genera  and  species  of  Medusae.  Some  of  them 
are  statocysts.  Others  suggest  a  visual  function,  and  in 
the  Cubomedusae  there  are  fairly  well  developed  eyes. 

Various  annelids  show  response  to  changes  in  light  in- 
tensity, the  leech  Clepsine,  for  example :  the  slightest 
shadow  cast  on  the  surface  of  the  water  in  a  dish  where 
these  animals  are  resting  quietly  will  cause  them  to  reach 
up  and  sway  from  side  to  side  in  an  apparent  search  for 
prey  (785).  On  the  other  hand  Gee  (256)  says  of  the  leech 
Dina  microstoma  that  the  casting  of  a  shadow  on  it  makes 
it  contract.  This  is  apparently  the  more  primitive  and 
the  more  common  type  of  response  to  a  change  in  light 
intensity.  Dina  contracts  in  just  the  same  way  when  me- 
chanically jarred,  but  a  difference  in  the  physiological  pro- 
cess involved  is  indicated  by  the  fact  that  these  leeches 
get  used  to  repeated  shadows,  and  cease  to  respond,  much 
more  quickly  than  they  get  used  to  repeated  jars.  When 
the  earthworm  has  partially  emerged  from  its  burrow, 
and  has  its  tail  still  inserted,  a  flash  of  light  will  produce 


140  The  Animal  Mind 

quick  withdrawal  into  the  burrow  (171,  327),  but  the  re- 
action time  to  light  is  much  longer  than  that  to  mechanidal 
stimulation.  The  part  of  the  earthworm's  body  affected 
by  the  light  also  influences  the  reaction.  Darwin  indeed  re- 
ported that  the  worms  withdrew  into  their  burrows  only 
when  light  fell  on  the  head  end  (171),  but  decapitated 
worms  were  found  by  Graber  to  respond  to  light  like  nor- 
mal ones,  only  less  strikingly  (266),  and  Yung  (833)  ob- 
tained evidence  that  sensitiveness  to  light  is  distributed 
over  the  body.  According  to  Hesse  the  anterior  end  of 
the  worm  is  most  sensitive,  the  tail  next,  and  the  middle 
region  least  (316).  Not  only  the  region,  but  the  amount 
of  body  surface  affected,  makes  a  difference.  When  the 
whole  length  of  the  worm  was  illuminated,  the  percentage 
of  reactions  was  to  that  obtained  where  the  front  third 
only  was  involved  as  26  to  10.2,  while  the  relative  occur- 
rence of  responses  where  the  middle  third  and  the  posterior 
third  alone  were  stimulated  is  represented  by  the  figures 
2.4  and  i  respectively  (552). 

In  many  of  the  marine  worms  well-developed  eyes  exist, 
although  not  such  as  are  capable  of  giving  clear  images. 
Their  function  seems  to  be  chiefly  that  of  receiving  stimuli 
from  shadows.  Many  tube-dwelling  worms  will  with- 
draw into  their  tubes  if  a  shadow  is  cast  upon  them  (285, 
321,  650). 

Turning  to  the  molluscs,  we  find  that  the  siphons  of  the 
Acephala,  which  are  projected  from  the  shell  to  take  in 
currents  of  water  containing  nourishment,  are  withdrawn 
in  response  to  sudden  darkening  in  some  cases,  to  sudden 
illumination  in  others,  and  in  still  other  instances  to  either 
(195,  520,  650).  The  danger  of  arguing  the  existence  of 
sensory  discrimination  from  structure  alone  is  well  shown 
in  the  case  of  snails,  for  although  many  of  them  have  eyes 


Sensory  Discrimination:   Vision  141 

of  some  degree  of  development,  these  very  species  have 
been  shown  to  be  devoid  of  sensitiveness  to  light  (836, 837). 

"Skioptic"  reactions,  or  reactions  to  shadows,  appear 
among  various  echinoderms.  The  sea-urchin  Centroste- 
phanus  longispinus,  for  instance,  which  lacks  even  a  rudi- 
mentary eye  spot,  will  when  a  sudden  shadow  falls  upon 
it  direct  its  spines  towards  the  shaded  side.  The  reaction 
time  involved  is  decidedly  longer  than  that  to  mechanical 
stimulation,  and  moreover,  although  pieces  of  the  animal 
will  react  to  the  latter,  responses  to  shadows  depend  on  keep- 
ing the  system  of  radial  nerves  intact.  (This  observation, 
according  to  Cowles  (155),  does  not  hold  for  the  sea-urchin 
Toxopneustes.)  Hence  Von  Uexklill,  who  made  the  above 
observations,  concluded  that  a  special  set  of  nerve  fibres 
is  concerned  in  photic  reactions  (735). 

Dubois  had  suggested  from  studies  on  the  mollusc 
P kolas  dactylus,  that  in  such  cases  the  pigment  changes 
which  occur,  under  the  influence  of  light,  over  the  surface 
of  the  body,  furnish  the  stimulus  (195),  but  Von  Uexkiill 
thinks  this  impossible,  as  the  light  reactions  occur  before 
the  pigment  changes  do.  This  migratory  pigment,  he  be- 
lieves, acts  merely  as  a  screen ;  the  source  of  excitation  for 
the  optic  fibres  may  lie  in  another  pigment  which  he  has 
extracted  and  found  very  sensitive  to  light  (735).  Cen- 
trostephanus,  according  to  Hess  (313),  shows  pigment 
changes  when  the  light  is  decreased  by  a  very  slight  amount, 
just  enough  to  be  perceptible  to  the  human  eye. 

Starfish  have  pigment  or  eye  spots  on  the  arm-tips. 
As  a  rule,  they  seek  light :  Romanes  (641)  and  Tiedemann 
(710)  report  that  the  light  reactions  are  abolished  if  the  eye 
spots  are  removed.  MacCurdy  (453)  finds,  however,  that 
in  Asterias  forbesii  the  light  reactions  are  independent  of 
eye  spots:  Cowles  (156)  has  shown  that  Echinaster  will 


142  The  Animal  Mind 

react  to  light  without  eye  spots,  although  some  evidence  of 
dependence  on  the  sense  organ  is  indicated  by  the  fact 
that  the  response  is  slower ;  Plessner  (607)  holds  that  skin 
sensitiveness  is  responsible  for  reactions  to  light  intensity, 
and  that  the  eye  spots  enable  the  animal  to  respond  to  the 
direction  of  the  light;  Cowles  (155),  again,  observes  that 
when  pieces  of  starfish  and  sea  urchins  are  cut  off,  their 
tentacles  and  suckers  still  move  in  response  to  the  casting 
of  a  shadow. 

Among  Crustacea,  which  are  provided  with  a  peculiar 
visual  organ,  the  compound  eye,  to  be  described  later,  the 
chief  function  of  the  eye  seems  to  be  that  of  responding  to 
shadows  and  movements.  Bateson,  watching  shrimps  and 
prawns,  noted  that  they  apparently  could  not  see  their 
food  when  it  had  been  taken  from  them  and  lay  near  at 
hand,  but  quickly  raised  their  antennae  when  an  object 
was  passed  between  them  and  the  light  (24).  The  little 
fairy  shrimp,  Branchipus,  will  stop  swimming  as  soon  as 
the  edge  of  a  shadow  falls  upon  it.  Skioptic  reactions  in 
the  family  of  Cirripedia,  to  which  the  barnacles  belong, 
were  noted  by  Pouchet  and  Joubert  in  1875,  as  well  as  the 
fact  that  those  individuals  which  were  attached  to  rocks, 
where  a  sudden  shadow  might  mean  danger,  reacted,  while 
those  attached  to  floating  objects,  and  therefore  normally 
exposed  to  light  fluctuations,  did  not  (615). 

When  we  come  to  animals  with  well-developed  eyes, 
the  specialized  response  to  changes  in  light  intensity  gives 
place  to  reactions  involving  the  use  of  a  more  or  less  ade- 
quate image  of  the  stimulus  object.  But  as  we  have  seen, 
the  most  primitive  type  of  reaction  to  a  sudden  change  of 
light  intensity  is  the  checking  or  inhibiting  of  the  animal's 
movements.  This  type  of  reaction  is  called  by  Loeb 
"sensibility  to  difference."  In  many  cases  its  relative 


Sensory  Discrimination:   Vision  143 

slowness,  as  compared  with  similar  reactions  to  other 
stimuli,  or  its  dependence  on  a  special  organ  or  region 
of  the  body,  give  evidence  that  it  is  accompanied  by  a 
specific  sensation  quality. 

§  39.   The  Continuous  Action  of  Light:  Photokinesis 

When  light  stimulates  not  through  its  change  of  intensity, 
but  through  its  action  as  a  constant  force,  its  effects  are 
apparently  of  two  kinds.  One  of  these  is  the  phenomenon 
which  Loeb  (433)  calls  the  tropism,  from  the  Greek  word 
meaning  "to  turn."  In  the  tropism,  the  organism  takes  up 
a  definite  position  with  reference  to  the  action  of  a  force. 
The  phenomenon  is  therefore  connected  rather  with  the 
spatial  aspect  of  the  sense  of  sight  than  with  its  qualitative 
aspect,  and  we  shall  consider  it  in  a  later  chapter.  The 
other  type  of  effect  caused  by  light  as  a  constant  stimulus 
is  that  it  stimulates  or  inhibits  the  general  activities  of  the 
animal.  Thus  some  animals  are  restless  in  strong  light, 
others  in  darkness.  As  a  result  of  this  influence,  the  former 
tend  to  form  collections  in  the  dark,  where  they  remain 
quietly ;  the  latter  in  the  light.  This  influence  of  a  certain 
intensity  of  light  to  stimulate  to  activity  we  may  call  its 
kinetic  effect,  or  photokinesis.  The  coelenterate  Hydra, 
for  example,  has  a  disposition  to  come  to  rest  in  the  more 
illuminated  parts  of  the  vessel  containing  it  (719,  791). 
Very  strong  light,  however,  makes  it  wander  about  until 
it  happens  to  reach  a  more  shaded  region.  Thus  if  the 
animal  is  subjected  to  light  either  above  or  below  a  certain 
"optimum"  of  intensity,  it  is  restless.  A  vague  uneasiness 
is  the  kind  of  psychic  accompaniment  to  this  behavior 
most  naturally  suggested.  Since  repeated  strong  mechani- 
cal stimulation  also  will  make  the  animal  wander,  nothing 


144  The  Animal  Mind 

points  to  the  existence  of  a  specific  visual  quality  in  this 
consciousness. 

The  medusa  Gonionemus  is  less  active  in  darkness  than 
in  light,  and  comes  to  rest  in  darkened  regions,  where  it 
thus  tends  to  collect  (802).  Such  collections  are  evidently 
not  due  to  a  definite  choice  on  an  animal's  part. 

On  the  planarian,  the  general  effect  of  light  stimulation 
is  kinetic ;  it  comes  to  rest  in  the  shaded  portions  of  a  ves- 
sel (20,  425,  429,  317).  Decapitated  and  hence  eyeless 
planarians  respond  to  light,  but  their  reactions  are  de- 
layed (432) ;  thus  there  is  a  certain  amount  of  dependence 
on  the  visual  organ. 

Photokinetic  effects  seem  to  be  common  among  insects, 
many  of  which,  the  house  fly,  for  example,  and  the  mason 
wasp  (728),  are  active  in  light  and  sluggish  in  darkness. 
These  animals  are  naturally  so  much  more  active  than  Hy- 
dra and  planarians  that  we  do  not  find  them  forming  col- 
lections in  the  regions  where  they  can  rest ;  they  seem  able 
to  continue  in  rapid  motion  for  long  periods,  and  it  is  rather 
a  pleasurable  than  an  uneasy  activity  that  is  suggested  by 
the  aerial  dances  of  insects  in  the  sun. 


§  40.    The  Problem  of  Visual  Qualities:  Invertebrates 

It  is  a  well-known  fact  that  when  a  human  being  with 
normal  vision  looks  at  the  band  of  spectral  colors,  the 
band  appears  brightest  to  him  in  the  region  of  the  yellow. 
Yellow  rays,  that  is,  produce  most  effect  on  the  normal  hu- 
man retina.  They  are  also  the  most  intense  rays  in  sun- 
light. Now  if  a  totally  color-blind  human  being  looks  at 
the  spectrum,  he  sees  it  as  a  band  of  different  grays,  the 
brightest  gray  being  not  in  the  yellow  region  but  in  the 
yellow-green;  that  is,  it  has  been  shifted  towards  the 


Sensory  Discrimination:   Vision  145 

violet  end.  He  also  sees  in  place  of  the  red  a  gray  darker 
than  the  brightness  of  red  to  the  normal  eye  would  lead 
one  to  expect.  This  altered  distribution  of  brightness 
in  the  spectrum  occurs  for  the  normal  eye  also,  under 
very  faint  illumination :  in  twilight  the  spectrum  looks 
to  the  normal  eye  just  as  it  does  to  the  totally  color-blind 
eye,  a  band  of  grays  brightest  in  the  yellow-green  region. 
If  we  had  no  other  means  of  deciding  whether  or  not  a 
man  was  color-blind,  we  should  take  as  evidence  of  color- 
blindness the  fact  that  for  him  the  brightest  region  of  the 
spectrum  lay  in  the  yellow-green  rather  than  the  yellow,  in 
ordinarily  bright  light.  It  is  therefore  of  some  impor- 
tance to  the  problem  of  color  vision  in  the  lower  animals 
to  find  how  strongly  the  light  rays  of  various  wave-lengths 
affect  them.  But  we  must  bear  in  mind  that  for  the  lower 
animals  it  is  impossible  to  conclude  color-blindness  from 
the  fact  that  the  brightness  values,  that  is,  the  effective 
intensities,  of  the  different  colors  are  what  they  would  be 
for  a  color-blind  human  being.  Just  this  unsafe  inference 
is,  however,  drawn  by  certain  authorities. 

In  plants,  the  maximum  effect  of  colored  light  is  exerted 
by  the  rays  at  the  violet  end :  violet,  indigo,  or  blue.  The 
problem  has  been  investigated  for  microscopic  animals  by 
an  arrangement  such  that  two  beams  of  light  fall  on  the 
organism  at  right  angles  to  each  other.  Now  if  the  or- 
ganism has  a  tendency  either  to  seek  or  to  avoid  light, 
and  if  the  two  beams  are  of  equal  intensity,  the  animal  will 
move  on  a  diagonal  between  the  two  beams.  If  either 
ray  has  a  stronger  effect  than  the  other,  the  course  of  an 
animal  which  seeks  light  will  be  inclined  towards  the  more 
effective  beam;  that  of  an  animal  which  avoids  light, 
towards  the  less  effective  beam.  If  the  two  beams  are  of 
different  colors,  it  will  thus  be  possible  to  test  the  stimu- 


146  The  Animal  Mind 

la  ting  efficiency  of  differently  colored  rays.  Mast  (474), 
using  this  method,  found  that  some  animal  forms,  such  as 
the  larvae  of  the  blowfly,  are  most  strongly  stimulated  by 
that  region  of  the  spectrum  which  acts  most  strongly  on 
the  color-blind  human  being;  others,  such  as  the  earth- 
worm and  the  larvae  of  the  worm  Arenicola,  were  most 
responsive  to  blue,  as  plants  are.  We  shall  later  note  the 
significance  which  Loeb  ascribes  to  resemblances  between 
plant  and  animal  responses  to  light. 

Amoeba,  which  as  we  have  seen  reacts  to  a  change  of 
light  intensity  by  a  checking  of  movement  at  the  point 
affected,  appears  when  tested  by  light  passed  through 
differently  colored  filters  to  react  in  the  majority  of  cases 
most  markedly  to  blue,  although  there  are  individual 
variations :  some  individuals  respond  most  definitely  to 
violet,  others  to  green  or  yellow,  and  still  others  to  red 
(467).  The  difference  is  merely  in  degree  of  response,  and 
we  can  infer  nothing  about  a  qualitative  differentiation  of 
conscious  accompaniments.  In  Hydra,  which  comes  to 
rest  in  moderately  illuminated  regions,  blue  and  green  light 
seem  to  be  a  better  substitute  for  white  light  than  are  red 
and  yellow  (791).  Schmid  (661)  says  that  red  and  yellow 
affect  the  sea-anemone  Cereactis  aurantiaca  differently  from 
blue  and  green,  but  does  not  state  wherein  the  difference 
consists. 

Graber  (266)  attempted  to  test  the  color  discriminations 
of  a  great  many  different  animal  forms  by  observing  their 
preferences  for  differently  colored  lights.  As  we  have  al- 
ready seen,  where  an  animal  displays  no  preference  or  choice 
between  two  stimuli,  it  by  no  means  follows  that  the 
stimuli  are  not  discriminated :  they  may  produce  differ- 
ent sensation  qualities  which  are  equally  agreeable  or 
disagreeable  to  the  animal.  When  earthworms  were  the 


Sensory  Discrimination:   Vision  147 

subject  of  tests  by  the  Preference  Method,  it  was  found 
that  if  a  choice  was  offered  between  two  compartments, 
one  illuminated  with  diffuse  daylight,  the  other  dark, 
and  if  the  number  of  worms  in  each  compartment  was 
counted  at  the  end  of  every  hour,  those  in  the  darkness 
were  on  the  average  5.2  as  many  as  those  in  the  light.  If 
ground  glass  was  substituted  for  the  dark  screen,  making 
the  compartment  under  it  about  half  as  light  as  the  other, 
the  number  in  the  lighter  compartment  was  about  .6  of  the 
number  in  the  darker,  though  still  moderately  light,  com- 
partment, showing  that  the  worms  were  sensitive  to  com- 
paratively small  differences  in  intensity.  When  colored 
glasses  were  placed  over  the  compartments,  the  follow- 
ing results  were  obtained :  the  worms  preferred  red  to  blue 
even  when  the  former  was  much  lighter  than  the  latter 
to  the  human  eye;  they  preferred  green  to  blue  under 
similar  conditions,  and  red  to  green.  They  emphatically 
preferred  white  light  from  which  the  ultra-violet  rays  had 
been  subtracted  to  ordinary  white  light,  6.7  times  as  many 
being  found  in  a  compartment  covered  by  a  screen  imper- 
vious only  to  ultra-violet  rays.  It  would  thus  appear  that 
in  determining  avoidance,  blue  light  is  the  most  effective ; 
on  the  other  hand,  Yung  (833)  finds  the  effect  of  colored 
rays  on  the  earthworm  to  be  proportional  to  their  intensity, 
the  green  and  yellow  regions  of  the  spectrum  being  most 
effective. 

It  is  thus  clear  that  when  an  animal  discriminates  between 
rays  of  different  colors,  the  discrimination  may  be  based 
merely  on  the  intensity  of  the  rays,  either  in  themselves  or 
in  the  effect  which  they  have  on  the  organism,  rather  than 
on  their  wave-length  or  color.  Minkiewicz  offers  as  evi- 
dence of  true  color  discrimination  in  a  Nemertean  worm, 
Linens  ruber,  the  fact  that  he  could  alter  its  reactions  to 


148  The  Animal  Mind 

colored  light  while  its  response  to  white  light  remained 
unchanged.  When  placed  in  diluted  sea  water,  the  animal 
would,  after  a  day,  direct  itself  toward  violet  rays,  although 
still  negative  in  response  to  white  light.  On  the  fourth 
day  the  ordinary  " chromotropism "  was  restored;  that  is, 
the  worm  sought  red  rays.  After  two  or  three  weeks  of 
life  in  the  diluted  sea  water,  on  being  restored  to  ordinary 
sea  water  the  worm  again  showed  inverted  chromotropism, 
becoming  "  positive  "  to  the  violet  rays,  while  still  "  nega- 
tive "  to  white  light.  Moreover,  intermediate  stages  in  the 
passage  from  the  red-  to  the  violet-seeking  phase  were  ob- 
served ;  a  stage  where,  still  positive  to  red,  the  animal  ceased 
to  distinguish  red  from  yellow,  and  others  where  it  sought 
violet,  but  had  become  indifferent  to  green  and  yellow. 
These  stages  lasted  for  several  hours,  but  corresponding 
ones  were  not  observed  during  the  passage  from  the  violet 
phase  back  to  the  red  phase :  perhaps  they  occurred  too 
rapidly  to  be  noted  (493). 

Hess  (314),  on  the  other  hand,  concludes  the  total  color- 
blindness of  the  marine  worm  Serpula  from  the  fact  that 
when  tested  by  the  direction  in  which  it  turned  when  sub- 
jected to  light  passing  through  differently  colored  glass,  it 
showed  evidence  that  the  yellow-green  had  most  effect, 
and  that  the  effectiveness  diminished  rapidly  towards  red, 
slowly  towards  violet :  in  other  words,  that  the  brightness 
effect  of  the  colors  was  like  that  shown  in  the  case  of  a 
color-blind  human  being. 

Hess  (306)  also  studied  the  comparative  effectiveness  of  dif- 
ferent colored  rays  on  the  eyes  of  cephalopod  mollusks  by 
measuring  with  a  special  instrument  the  degree  of  expan- 
sion or  contraction  of  the  pupil  produced  by  the  various 
colors.  He  found  that  the  yellow  and  green  rays  produce 
much  more  effect  than  the  red  and  violet  rays.  Since  this 


Sensory  Discrimination:   Vision  149 

is  true  also  of  the  color-blind  human  eye,  he  argues  that  the 
animals  tested  are  totally  color-blind.  He  holds,  in  fact, 
that  all  invertebrate  animals  are  totally  color-blind,  on 
the  same  evidence.  The  feet  of  starfish  belonging  to  the 
Astropectinidae  are,  he  says,  very  sensitive  to  light:  red 
light  has  little  effect  on  them,  blue  and  green  light,  even 
when  much  darker  than  red  to  normal  human  vision,  de- 
cidedly more  effect,  ^^^_^  ^^  ^ 
as  they  would  have  \  .  /^S^^?3^^^\  / 

,  ^^L^T^^^Cw  \f 

for  a  totally  color- 
blind human  being. 
The  same  results 
appear  in  the  case 
of  the  sea-urchin. 

A      human     being's      FIG.  10. —  Daphnia.    at,  antenna;  all,  antennule; 

sensitiveness  to  °c'^'  AteY«te- 

light  is  increased  when  he  remains  in  darkness  for  some 
time.  This  effect,  called  darkness  adaptation,  appears 
according  to  Hess  in  the  Astropectinidae  (313). 

The  problem  as  to  whether  light  of  different  colors  pro- 
duces different  sensations  in  the  crustacean  consciousness 
was  the  subject  of  experiments  a  number  of  years  ago,  in 
which  the  Preference  Method  was  used.  Lubbock  (442, 
443)  arranged  to  have  a  sunlight  spectrum  thrown  on  a 
long  trough  containing  Daphnias,  tiny  crustaceans  belong- 
ing to  the  lowest  subclass,  that  of  the  Entomostraca  (Fig. 
10).  Daphnia  is  ordinarily  positive  in  its  response  to  light, 
that  is,  it  seeks  light.  At  the  end  of  ten  minutes  glass 
partitions  were  slipped  across  the  trough  at  the  approxi- 
mate dividing  lines  between  the  spectral  colors.  The  num- 
ber of  animals  in  each  compartment  was  then  counted. 
The  experiment  was  repeatedly  performed,  and  the  greatest 
number  was  always  found  in  the  yellow-green  region.  Bert 


150  The  Animal  Mind 

obtained  similar  results  with  the  use  of  an  electric  light 
spectrum ;  but  besides  throwing  all  the  colors  at  once  upon 
the  vessel,  he  allowed  each  color  to  act  separately  through 
a  narrow  opening,  and  noted  the  speed  of  the  positive 
response  produced.  That  the  "preference"  shown  for 
yellow-green  light  is  not  a  matter  of  color  vision,  but  of 
response  to  the  greater  intensity  of  the  light  in  this  region 
of  the  spectrum,  was  suggested  by  Bert  (46),  and  Merej- 
kowsky  showed  that  the  larvae  of  Balanus  and  Dias  longi- 
remis  manifested  no  color  preference  when  the  colors  were 
made  of  equal  intensity  (484) .  Lubbock  attempted  to  prove 
the  existence  of  qualitative  as  distinguished  from  intensive 
discrimination  by  various  modifications  of  the  experiment, 
but  without  entirely  conclusive  results  (444,  pp.  221  ff.). 
Yerkes,  working  on  Simocephalus,  a  form  closely  related 
to  Daphnia,  found  that  when  a  gaslight  spectrum  was 
used,  the  animals  collected  in  the  red-yellow  region,  that 
of  greatest  intensity  for  such  light ;  and  that  if  this  region 
had  its  intensity  diminished  by  a  screen  of  India  ink  or 
parafnne  paper,  the  crustaceans  moved  out  of  it  (799). 
This  seemed  strong  evidence  that  the  apparent  color 
reactions  of  these  animals  were  really  responses  to  differ- 
ences in  the  intensity  of  the  light.  Hess  (306)  studying 
the  movements  of  the  eyes  of  Daphnia  when  subjected  to 
light  of  different  colors,  finds  another  case  of  total  color- 
blindness, and  Erhard  (207)  gets  similar  results  on  Simo- 
cephalus when  the  light  is  reflected  from  colored  surfaces. 
Nevertheless,  there  is  evidence  that  colored  rays  have  an 
effect  on  these  crustaceans  that  is  not  wholly  dependent 
on  their  intensity ;  evidence,  that  is,  in  favor  of  color  vision. 
When  Daphnias  have  been  kept  for  some  time  in  light  of 
a  certain  intensity,  an  increase  in  the  intensity  makes  them 
avoid  light,  while  a  decrease  in  intensity  makes  them  seek 


Sensory  Discrimination:   Vision  151 

light.  But  if  a  blue  screen  is  interposed  between  the 
light  and  the  animal,  hi  spite  of  the  fact  that  the  intensity 
is  thereby  diminished,  the  Daphnias  avoid  it;  if  yellow 
light  is  added  to  white  light,  in  spite  of  the  fact  that  the 
intensity  of  the  light  is  thereby  increased,  the  Daphnias 
seek  it.  These  results  were  obtained  by  Von  Frisch  (248), 
who  is  as  determined  to  find  color  vision  in  invertebrates 
as  Hess  is  to  disprove  it.  Ewald  (213)  reports  that  of  the 
Daphnias  under  his  observation  one  group  sought  the  light, 
which  was  in  this  case  most  effective  in  the  green-yellow 
regions,  but  that  another  group  avoided  light,  and  for  these 
the  most  effective  region  was  the  blue-violet,  so  the  effect 
of  colored  rays  was  independent  of  intensity.  He  reports, 
however,  that  certain  colored  rays  could  be  replaced  by 
colorless  rays  without  affecting  the  responses  of  the  Daph- 
nias ;  these  rays  were  the  red  and  green  ones.  He  there- 
fore concludes  that  Daphnia  is  not  totally  color-blind,  but 
red-green  blind.  This,  as  we  shall  see,  is  Von  Frisch's 
belief  with  regard  to  certain  other  invertebrates.  Ewald 
thinks  he  has  also  evidence  in  the  case  of  Daphnia  of  si- 
multaneous contrast  and  successive  contrast,  such  as  human 
vision  shows.  The  successive  effect  (negative  after-images) 
occurs  for  both  color  and  brightness  stimuli,  and  is  shown 
by  the  fact  that  the  animals  reverse  their  reaction  to  the 
same  white  light  according  as  they  have  been  exposed  pre- 
viously to  white  (or  blue)  light,  or  to  darkness  (or  yellow 
light).  Simultaneous  contrast  Ewald  concludes  from  the 
observation  that  when  the  region  surrounding  a  constant 
stimulus  light  is  brightened,  the  reaction  of  the  animals 
tends  to  become  positive,  that  is,  they  move  towards  the 
light;  darkening  the  surroundings  makes  them  move 
towards  the  same  light.  This  effect,  it  is  argued,  is  due  to 
the  stimulation  of  the  side  regions  of  the  eye :  now,  since 


152  The  Animal  Mind 

colors  have  no  special  influence  in  producing  these  simul- 
taneous contrast  phenomena,  Ewald  concludes  that  the 
side  regions  of  Daphnia's  eye,  like  those  of  our  own  eye,  are 
totally  color-blind.  All  of  which  seems  a  heavy  weight  of 
inference  to  depend  from  rather  slender  evidence. 

That  Daphnia  seeks  a  region  affected  by  the  ultra-violet 
rays  of  the  spectrum  in  preference  to  darkness,  although 
the  two  look  alike  to  our  eyes,  was  shown  by  Lubbock  (444) . 
On  the  other  hand,  Loeb  (431)  and  Moore  (502)  report 
that  the  action  of  ultra-violet  rays  instantly  makes  Daphnia 
avoid  the  light  containing  them,  and  it  appears  that  these 
rays,  which  are  without  effect  on  the  human  eye,  exert 
their  influence  through  the  eye  of  Daphnia,  since  individuals 
without  eyes  are  unaffected  by  them  (300). 

Adaptation  to  darkness  apparently  takes  place  in  the 
eye  of  Daphnia,  for  individuals  which  have  been  a  long 
time  in  darkness  will  respond  to  a  lower  intensity  of  light 
than  those  which  have  been  long  exposed  to  illumination 
(174).  Experiments  on  the  effect  of  intermittent  and 
continuous  lights  of  equal  intensity  on  the  movements  of 
the  Daphnia  eye  indicate  that  the  Talbot-Plateau  Law, 
according  to  which  such  lights  are  identical  in  effect, 
holds  for  the  vision  of  this  crustacean  as  for  the  human  eye 
(212).  It  is  this  law  which  enables  us  to  measure  the  grey 
produced  by  a  rapidly  revolving  disk  of  black  and  white 
sectors  as  equal  in  brightness  to  the  amount  of  light  re- 
flected by  the  sectors  at  rest. 

Experiments  on  the  reactions  of  the  crayfish,  which  has 
a  moderately  strong  tendency  to  avoid  light,  show  that  when 
light  falls  vertically  through  red  glass  the  animal  seeks  it 
in  preference  to  darkness :  no  marked  preference  is  indi- 
cated when  the  light  is  passed  horizontally  through  red 
glass  (40).  No  clear  evidence  of  color  discrimination  ap- 


Sensory  Discrimination:   Vision  153 

pears  here.  Hess  (306),  of  course,  holds  that  all  crus- 
taceans are  totally  color-blind,  arguing  from  his  results  on 
the  relative  stimulating  effect  upon  them  of  different 
spectral  colors.  Minkiewicz  (494,  495),  on  the  other 
hand,  believes  he  has  evidence  of  colof  discrimination  in 
certain  crabs.  The  hermit  crabs,  for  instance,  are  natu- 
rally attracted  to  light,  but  when  subjected  to  colored  lights 
they  do  not  seek  them  in  the  order  of  their  intensity. 
Green  is  the  most  attractive  color,  violet  next;  then  the 
order  is  "blue,  yellow,  red,  and  black."  He  finds  it  possible 
with  crabs,  as  with  worms  (see  page  147),  to  reverse  the 
response  to  white  light  without  reversing  the  response  to 
color  (493).  Minkiewicz's  most  remarkable  observations 
were  made  on  certain  crabs  (Maia)  which  have  the  instinct 
possessed  by  many  crab  species  of  attaching  to  their  shells 
foreign  objects,  bits  of  seaweed  and  the  like,  serving  the 
purpose  of  making  them  less  conspicuous  in  their  ordinary 
environment.  When  these  crabs  are  kept  for  some  time  in 
an  aquarium  lined  with  a  certain  color,  their  subsequent 
behavior  is  modified  in  two  ways,  (i)  On  being  given  bits 
of  paper  some  of  which  are  colored  like  the  aquarium,  while 
others  are  of  a  different  color,  the  crabs  select  for  deco- 
rative purposes  the  bits  that  match  their  surroundings. 
(2)  When  placed  in  another  aquarium  offering  a  choice 
between  two  compartments,  one  with  walls  matching 
those  of  the  tank  they  have  left,  the  other  with  differently 
colored  walls,  the  crabs  choose  the  former.  Two  American 
investigators  have  performed  experiments  similar  to  these. 
Pearse  (567)  fails  to  get  any  evidence  that  when  crayfishes, 
spider  crabs,  crab  spiders  and  caddis  fly  larvae  are  kept 
in  colored  boxes  they  develop  any  tendency  to  choose  later 
an  environment  of  the  same  color.  On  the  other  hand, 
Stevens  (693) ,  working  with  a  Pacific  coast  crab  which  has 


154  The  Animal  Mind 

the  decorating  instinct,  finds  that  it  does  acquire  such  a 
tendency,  but  that  it  does  not  learn  to  decorate  itself  with 
colors  matching  its  surroundings.  The  acquired  "chromo- 
tropism,"  or  tendency  to  seek  a  certain  color,  in  crabs  might 
be  interpreted  as  merely  a  response  to  the  brightness  of  the 
colors,  not  to  their  color  as  such;  that  is  the  crabs  may 
after  all  be  totally  color-blind,  seeing  the  colors  as  grays. 
Stevens  found  indications  that  green  comes  nearest  to 
white  light  in  its  effect  on  the  animals,  by  noting  the 
promptness  and  accuracy  with  which  they  faced  the  light. 

Experiments  have  been  made  on  color  discrimination  in 
spiders:  some  by  the  Preference  Method,  where  the 
spiders  showed  an  inclination  for  red  when  offered  a  choice 
of  compartments  illuminated  through  red,  green,  blue, 
and  yellow  glass  (570) ;  others  by  attempting  to  form  an 
association  between  paper  of  a  certa'n  color  and  the  spider's 
nest.  This  latter,  containing  eggs,  was  surrounded  with 
colored  paper,  and  when  a  spider  had  become  accustomed 
to  going  in  and  out  over  the  paper,  another  color  was  sub- 
stituted, and  a  false  nest  made  in  another  place,  surrounded 
by  the  original  strips  of  paper.  The  spider  under  these 
circumstances  showed  some  confusion  and  tendency  to  go 
to  the  false  nest  It  is  obvious  that  this  method  takes  no 
account  of  the  possibility  that  the  spider  was  reacting  only 
to  the  intensity  of  the  colored  rays  and  not  to  their  color 
as  such  (571). 

On  the  color  sense  of  insects  there  are,  first,  the  old  ex- 
periments of  Graber  by  the  Preference  Method,  whose  most 
definite  result  was  to  show  that  positively  photo  tropic, 
that  is,  light-seeking,  insects  prefer  colors  containing  the 
ultra-violet  rays,  while  the  negatively  phototropic  or  light- 
avoiding  ones  prefer  red,  from  which  these  rays  are  absent. 
No  proof  that  the  discriminations  were  made  on  the  basis 


Sensory  Discrimination:   Vision  155 

of  color  proper  rather  than  brightness  was  forthcoming 
(267).  Similar  observations  were  made  by  Lubbock  on 
ants,  which  in  their  underground  life  are  negatively  pho- 
totropic,  the  eggs  and  larvae  apparently  needing  darkness 
in  order  to  develop,  but  on  their  foraging  expeditions  are 
comparatively  indifferent  to  light.  They  showed  a  pref- 
erence for  red  when  tested,  and  a  tendency  to  avoid  the 
ultra-violet  rays,  so  marked  that  they  preferred  bright  day- 
light from  which  these  rays  had  been  extracted  by  chemi- 
cal screens,  to  darkness  that  contained  the  ultra-violet 
rays  (441,  pp.  207  ff.).  Graber  suggested  that  the  ultra- 
violet rays  produce  a  skin  sensation  in  the  ants;  but 
Forel  agrees  with  Lubbock  that  the  effect  is  visual,  because 
he  found  that  varnishing  the  eyes  made  the  ants  indiffer- 
ent to  ultra-violet  (231).  Ants  of  the  family  Lasius  seem 
to  be  normally  insensitive  to  these  rays  (235).  It  is  just 
possible,  then,  that  a  visual  sensation  of  quality  wholly 
foreign  to  our  experience  may  accompany  the  action  of 
ultra-violet  rays  on  insects.  Loeb  has  noted  that  the 
relative  effect  of  violet  and  ultra-violet  vibrations,  as 
compared  with  that  of  the  rest  of  the  spectrum,  is  greater, 
the  less  developed  the  visual  organ  (419).  Termites,  which 
seek  darkness,  prefer  red  to  blue  colored  glass  (6). 

Lubbock's  experiments  on  the  color  sense  of  bees  are 
more  to  the  point  than  those  on  ants,  for  they  were  made 
not  by  the  Preference  Method,  but  by  associating  a  color 
with  food.  No  precaution,  however,  was  taken  against 
the  brightness  error.  He  found  that  bees  which  had  eaten 
honey  from  blue  paper  would  pick  out  the  blue  pieces  from 
a  number  of  differently  colored  papers,  whose  positions 
were  altered  during  the  experiments  (441).  Forel  got 
similar  results,  and  reports  that  a  bumblebee  thus  trained 
selected  all  the  blue  objects  in  the  room  for  special  ex- 


156  The  Animal  Mind 

animation  (231).  Lubbock's  tests  with  wasps  gave  nega- 
tive results.  Lovell  (439)  and  Turner  (725)  also  infer 
color  vision  in  the  honey-bee  from  its  ability  to  pick  out 
objects  of  the  same  color  as  that  on  which  it  has  recently 
found  food :  the  former  takes  no  account  of  the  brightness 
error,  while  the  latter  holds  that  it  has  been  sufficiently 
eliminated  by  the  fact  that  the  color  identifications  were 
made  by  the  bees  under  varying  lights  and  shades  out  of 
doors.  This,  however,  is  probably  an  inadequate  pre- 
caution. Von  Frisch  (246)  offers  more  convincing  evi- 
dence of  color  vision  in  the  bee,  and  thinks  he  has  indica- 
tions that  bees  are  red-green  color-blind.  His  experiments 
were  performed  in  the  open  air.  Having  trained  the  bees 
to  come  to  strips  of  yellow  paper,  on  which  food  was  placed, 
he  mingled  such  strips,  without  food,  among  strips  of  thirty 
different  shades  of  gray.  The  bees,  he  reports,  were  able 
to  make  the  discrimination,  and  to  do  equally  well  when 
blue  was  used :  they  failed,  however,  with  red,  confusing 
red-violet  with  blue,  and  dark  red  with  dark  gray.  In 
another  article  (247)  he  says  that  a  certain  bluish  green 
also  was  confused  with  gray.  This  general  method,  where 
a  large  range  of  grays  is  used  and  an  animal  proves  capable 
of  discriminating  a  color  from  any  or  all  of  them,  is  the 
best  way  of  eliminating  the  brightness  error.  The  use  of 
colored  papers  in  experiments  on  color  vision  in  animals 
is  open  to  criticism  unless  some  precaution  is  taken  against 
the  possibility  that  something  in  the  surface  texture  or 
grain  of  the  papers  may  help  the  animal  to  distinguish. 
This  possibility  Von  Frisch  guarded  against  by  varnishing 
the  papers,  a  proceeding  which  did  not  affect  the  behavior 
of  the  bees. 

Hess  (312,  315),  anxious  to  defend  his  theory  of  the  total 
color-blindness  of  all  invertebrates,  repeated  Von  Frisch's 


Sensory  Discrimination:   Vision  157 

experiments  and  could  not  confirm  his  results.  Using  col- 
ored lights  of  measured  intensities  and  studying  their  effect 
in  causing  the  bees  to  collect  under  them,  he  compared 
these  effects  with  the  influence  of  the  various  colors  on  the 
reflex  contraction  of  the  pupil  in  human  beings.  He  found 
that  the  smallest  differences  in  intensity  to  which  the 
bees  reacted  were  those  just  perceptible  to  the  human  eye, 
and  that  the  relative  effect  of  different  colors  was  like  their 
relative  effect  on  a  color-blind  human  being.  He  reports 
similar  results  with  butterflies.  Hess  thinks  this  method, 
which  deals  with  reflexes,  superior  to  any  method  which, 
like  Von  Frisch's,  involves  learning  on  the  part  of  the  ani- 
mals. But  again  we  may  remind  ourselves  that  it  does  not 
follow  that  because  a  human  being  who  finds  the  yellow- 
green,  rather  than  the  yellow,  the  brightest  spectral  region, 
is  totally  color-blind,  therefore  an  animal,  especially  an 
invertebrate  animal,  the  chemical  substances  .in  whose 
eye  may  have  no  resemblance  to  those  in  the  human  eye, 
is  color-blind  if  it  shows  these  reactions  to  the  differ- 
ent regions  of  the  spectrum.  Hess's  method  is  defective 
just  because  it  deals  with  reflexes  whose  stimuli  are  inten- 
sity differences.  If  an  animal  is  capable  of  distinguishing 
both  intensity  differences  and  color  differences,  the  use  of 
reflexes  that  depend  on  the  former  is  a  poor  way  of  study- 
ing the  capacity  to  discriminate  the  latter. 

We  have  already  noted  the  dispute  as  to  how  far  visual 
sensations  in  general  are  involved  in  the  reactions  of  bees  to 
flowers,  and  have  seen  that  Plateau  maintains  their  relative 
unimportance  in  this  connection,  as  compared  to  smell. 
Besides  the  experiments  which  we  have  quoted  on  pp.  104  f ., 
he  adduces  the  facts  that  he  could  never  persuade  in- 
sects to  alight  upon  artificial  flowers,  though  these  were 
not  distinguishable  by  human  eyes  from  real  ones  (600- 


158  The  Animal  Mind 

602) ;  that  bees  show  no  preference  for  flowers  of  any  par- 
ticular color  (603) ;  and  that  they  often  make  errors,  in 
alighting  on  closed  buds,  seed  pods,  and  wilted  flowers, 
which  indicate  defective  vision  (605).  But  Josephine 
Wery  and  others  have  noted  that  bees  do  seek  artificial 
flowers  (778).  Even  Plateau  does  not  deny  that  an  insect 
may  perceive  flowers  from  a  distance,  "  whether  because  it 
sees  the  color  in  the  same  way  that  we  do,  or  because  it 
perceives  some  kind  of  contrast  between  the  flowers  and 
their  surroundings"  (603). 

Von  Buttel-Reepen  (114)  gives  one  or  two  instances  to 
show  that  the  color  perception  of  bees  is  sometimes  influ- 
ential in  helping  them  to  recognize  their  own  hives.  He 
reports  a  case  where  a  stock  of  bees  had  been  driven  from 
their  hive  and  scattered.  The  front  of  the  hive  was  blue. 
Some  of  the  bees  tried  to  find  their  way  into  other  hives, 
and  selected  for  their  efforts  those  which  had  blue  doors. 

It  will  be  remembered  that  Loeb  is  convinced  that 
the  relative  effect  of  the  different  regions  of  the  spec- 
trum on  invertebrate  animals  is  identical  with  the  effect 
on  plants ;  that  is,  strongest  for  the  violet  end  of  the  spec- 
trum. This  position  has  no  significance  for  the  problem 
of  color  discrimination,-  but  obviously  Loeb  and  Hess  are 
sharply  opposed  as  to  the  facts.  Recently  Gross  (271) 
has  used  colored  spectral  lights  of  carefully  equated  in- 
tensity, and  a  method  which  permits  measurement  of  the 
exact  amount  of  light  effect,  in  deflecting  an  animal  from 
its  course  of  movement.  Adult  blowflies,  fruit  flies,  and 
moths,  as  well  as  larvae,  were  used  as  subjects.  All  the 
lights  were  made  of  equal  intensity,  whereas  in  the  or- 
dinary daylight  spectrum  the  yellow  region  is  most  in- 
tense and  differences  of  intensity  exist  all  along  the  line. 
Under  these  conditions,  Loeb's  contention  was  confirmed 


Sensory  Discrimination:    Vision  159 

and  Hess's  overthrown  for  all  the  subjects  except  the  larvse 
of  the  blowfly ;  that  is,  the  insects  were  more  strongly  af- 
fected by  blue  than  by  yellow  or  green.  But  the  blowfly 
larva  was  more  strongly  affected  by  green  than  by  any 
other  colored  light.  It  responded,  in  other  words,  as  Hess 
and  his  pupil  Weve  (779)  had  found  it  to  do. 


§  41.   The  Problem  of  Visual  Qualities:  Amphioxus 
and  Fish 

The  vertebrate  eye  differs  in  origin  and  structure  from 
any  form  of  invertebrate  eye,  the  most  striking  difference 
in  structure  being  the  location  of  the  pigmented  layer  of 
the  retina  behind  the  nerve  fibre  layer,  a  location  which  is 
responsible  for  the  existence  of  the  blind  spot  in  the  verte- 
brate eye,  where  the  trunk  of  the  optic  nerve  breaks 
through  the  retinal  layers.  Another  point  of  unlikeness 
consists  in  the  fact  that  the  invertebrate  optic  nerves  do 
not  cross  on  their  way  to  the  brain,  while  in  the  verte- 
brates there  is  either  total  or  partial  crossing  of  the  fibres. 

The  reactions  of  Amphioxus  to  light  offer  as  chief  evi- 
dence that  they  are  accompanied  by  a  specific  sensation 
quality  the  fact  that  they  may  be  fatigued  independently 
of  other  reactions.  The  only  structures  suggesting  a  visual 
function  are  pigment  spots  on  the  back  near  the  head,  and 
other  pigment  spots  distributed  down  the  back.  Amphi- 
oxus makes  negative  responses  to  light,  especially  when  the 
light,  from  which  heat  rays  have  been  extracted  by  passing 
it  through  water,  is  directed  at  any  point  on  the  back,  the 
most  sensitive  region  lying  just  behind  the  eye-spot  (406, 
543).  Fatiguing  the  light  reactions  had  no  effect  on  re- 
sponse to  other  forms  of  stimulation  (543).  Attempts  to 
test  the  color  "preferences"  of  Amphioxus  by  illuminating 


160  The  Animal  Mind 

different  parts  of  a  trough  with  differently  colored  lights 
gave  negative  results  (406).  Hess  (306)  found  that  the 
maximal  effect  on  the  activity  of  Amphioxus  was  exerted 
by  the  yellow  and  green  rays,  the  red  and  violet  being  much 
less  effective;  hence  he  concluded  that  as  in  all  inverte- 
brates, so  in  this  rudimentary  vertebrate,  total  color-blind- 
ness exists. 

True  skin  sensitiveness  to  light  has  been  observed  in 
larval  lampreys,  which  will  give  negative  reactions  even 
when  the  optic  nerves  are  cut  (540),  and  in  cave-dwelling 
blind  fish  (201).  Parker,  however,  finds  no  other  fish  in 
which  it  exists,  although  it  is  quite  common  in  amphibians. 
He  therefore  reaches  the  conclusion  that  in  vertebrates, 
skin  sensitiveness  is  not  a  primitive  form  of  visual  sensi- 
bility, from  which  vision  by  the  eye  has  been  derived,  but 
a  "secondarily  acquired  peculiarity."  He  points  out  that 
the  fish  and  amphibians  which  show  it  are  freshwater 
animals,  whereas  the  primitive  vertebrates  were  certainly 
marine  (545). 

Among  the  many  animals  whose  supposed  color  prefer- 
ences Graber  tested  were  two  species  of  fish,  but  no  con- 
vincing proof  of  their  powers  of  color  discrimination  was 
obtained  (267).  Bateson  (25)  placed  food  on  differently 
colored  tiles,  and  observed  that  the  fish  picked  it  off  most 
readily  from  white  and  pale  blue,  and  least  readily  off  dark 
red  and  dark  blue;  which  establishes  little  save  that  the 
bait  was  probably  more  conspicuous  on  the  white  and 
pale  blue.  Professor  Bentley  and  the  writer  (757)  got  good 
evidence  that  the  common  brook  chub  could  distinguish 
between  red  and  green  paints,  by  training  it  to  bite  at  for- 
ceps to  which  red  sticks  were  attached,  and  to  refrain  from 
biting  at  similar  forceps  carrying  green  sticks.  The  pos- 
sibility of  guidance  by  smell  or  by  the  position  of  the  for- 


Sensory  Discrimination:   Vision  161 

ceps  was  ruled  out,  and  the  fish  could  identify  the  red 
forceps  whether  they  were  to  the  human  eye  darker  or 
lighter  than  the  green.  It  is  not,  however,  a  sufficient 
guard  against  the  brightness  error  to  use  human  judgments 
of  brightness  as  a  standard.  Reighard  (631),  similarly, 
trained  the  gray  snapper  to  avoid  minnows  dyed  in  certain 
colors  and  select  those  dyed  in  other  colors,  several  bright- 
nesses differing  to  the  human  eye  being  used,  but  the 
brightness  error  not  being  more  fully  eliminated.  Bauer 
(27)  believes  he  has  secured  evidence  that  fish  discriminate 
colors,  and  that  certain  fish  are  afraid  of  red,  but  the  gen- 
eral character  of  his  methods  and  conclusions  does  not 
inspire  confidence.  Hess  (304,  308)  is  convinced  that  the 
spectrum  is  seen  by  fishes  with  the  same  distribution  of 
brightnesses  that  is  characteristic  of  the  color-blind  human 
eye,  and  makes  the  inference,  which  we  have  previously  chal- 
lenged, that  total  color-blindness  must  exist  in  such  a  case. 
Von  Frisch  (245),  on  the  other  hand,  argues  that  fish  possess 
color  vision.  He  has  shown  their  ability  to  pick  out  a 
color  from  a  whole  series  of  grays.  He  points  to  the  fact 
that  in  the  spawning  season  many  fish  assume  bright 
colors  and  patterns ;  these,  he  urges,  must  have  some  in- 
fluence in  bringing  the  sexes  together  (243).  Hess  (310) 
in  opposition  to  this  points  out  that  such  colors  would  not 
be  visible  below  a  certain  depth  of  water ;  Von  Frisch  re- 
plies that  most  of  the  fish  which  show  them  spawn  in 
shallower  waters.  One  can  hardly,  however,  infer  color 
vision  from  the  existence  of  such  colors,  for  they  may  be 
only  incidental  effects  of  the  physiological  state  of  the 
animals,  and  without  any  influence  on  their  behavior.  A 
more  persuasive  line  of  argument  is  derived  from  the  way 
in  which  various  flatfish  change  their  markings  and  colors 
to  suit  the  ground  on  which  they  lie.  Von  Frisch  (240) 


1 62  The  Animal  Mind 

studied  this  phenomenon  in  the  case  of  the  fish  Phoxinus 
laems.  If  two  fish  whose  skins  are,  at  the  time,  of  equal 
brightness,  are  placed  one  on  a  yellow  ground,  the  other 
on  a  gray  ground,  and  these  grounds  are  properly  chosen  as 
to  brightness,  the  fish  will  not  alter  their  own  brightnesses, 
although  if  either  ground  is  made  lighter  or  darker  a  cor- 
responding change  occurs  in  the  skin  of  the  fish  lying  on 
the  altered  ground.  When  the  fish  remain  at  the  same 
brightness,  then,  it  may  be  inferred  that  the  "brightness 
values"  of  the  two  grounds  are  identical.  But  after  a  few 
hours,  it  will  be  found  that  the  fish  on  a  yellow  ground 
shows  a  yellow  stripe  which  does  not  appear  on  the  other 
fish  (see  also  277).  Mast  (475)  has  made  a  very  thorough 
study  of  this  phenomenon  in  the  case  of  the  flounders 
Paralichthys  and  Ancylopsetta.  These  fishes  become 
strikingly  bluish  on  blue  grounds,  greenish  on  green  grounds, 
and  so  forth,  adapting  themselves  to  blue,  green,  yellow, 
orange,  pink,  and  brown,  and  less  successfully  to  red.  The 
color  changes  are  brought  about  by  certain  pigment-con- 
trolling mechanisms  in  the  skin,  which  are  connected  with 
the  sympathetic  nervous  system.  But  the  color  stimulus 
acts  through  its  effect  on  the  eyes :  the  changes  do  not 
occur  if  the  eyes  are  covered.  Moreover,  the  effect  of  the 
stimulus  received  by  one  eye  is  modified  by  that  of  the 
stimulus  received  from  the  other  eye :  if  one  eye  is  on  a 
black  ground  and  the  other  on  a  white  ground,  the  skin 
becomes  gray.  Mast  succeeded  in  showing  that  the  rate 
at  which  alternating  black  and  white  sectors  must  follow 
each  other  in  order  to  fuse  into  a  continuous  gray  is  the 
same  for  the  eye  of  the  flounder  as  for  that  of  the  human 
being:  he  placed  the  fish  over  a  rotating  black  and 
white  disk  and  noted  the  speed  of  rotation  required  for  the 
fish  to  become  gray  instead  of  mottled  black  and  white. 


Sensory  Discrimination:   Vision  163 

Watson  (771)  does  not  think  these  observations  suffi- 
ciently prove  color  discrimination  on  the  part  of  the  fish. 
He  says,  "Ordinarily  we  mean  when  we  say  that  an  animal 
is  sensitive  to  difference  in  wave-length  that  such  stimuli 
play  a  role  in  the  adjustment  of  the  animal  to  food,  sexual 
objects,  shelter,  escape  from  enemies,  etc.  i.e.,  that  such 
stimuli  initiate  activity  in  arcs  which  end  in  the  striped 
muscles."  Because  the  changes  of  color  are  produced 
not  by  such  arcs,  but  by  the  sympathetic  nervous  system, 
Watson  thinks  color  vision  not  proved ;  "we  can  easily  con- 
ceive," he  says,  "of  mimicry  of  this  kind  taking  place  in 
an  animal  whose  retina  does  not  contain  the  physico-chemi- 
cal substances  .  .  .  necessary  to  initiate  response  to  differ- 
ences in  wave-length."  Since  the  changes  of  color  are  in- 
duced by  differences  in  wave-length  and  induced  through 
the  retina,  we  may  reply  that  it  does  not  seem  easy,  or  in 
fact  at  all  possible,  to  conceive  the  absence  of  such  photo- 
chemical substances  from  the  fish's  retina.  Moreover, 
Mast  finds  that  fish  which  have  thus  become  adapted  to 
a  given  color  will  seek  that  color :  this  is  an  activity  in- 
volving the  striped  muscles. 

On  the  whole,  the  weight  of  evidence  is  at  present  in 
favor  of  the  possession  of  color  vision  by  fish. 

§42.   The  Problem  of  Visual  Qualities:    Reptiles  and 
Amphibia 

Skin  sensitiveness  to  light  has  been  demonstrated  in 
certain  amphibians.  The  response  of  the  frog  to  light 
persists  when  the  animal  is  blinded,  although  in  the  nor- 
mal animal  the  eyes  are  involved  in  the  reaction,  since  it 
occurs  when  the  skin  is  covered  and  the  eyes  left  intact 
(405,  538).  The  skin  of  salamanders  also  is  sensitive  to 


164  The  Animal  Mind 

light  (196).  The  nature  of  the  "  dermal  light  sensation " 
remains  a  mystery.  It  can  hardly,  in  frogs,  be  a  painful 
irritation,  since  it  produces  a  positive  response;  and  it  is 
not  due  to  heat  rays,  for  it  occurs  when  these  are  inter- 
cepted by  passing  the  light  through  water.  As  Parker 
says,  radiant  heat  and  light,  "distinct  as  they  seem  to  our 
senses,  are  members  of  one  physical  series  in  that  they  are 
both  ether  vibrations,  varying  only  in  wave  length"  (538). 
While,  then,  the  nerve  endings  in  human  skin  are  sensi- 
tive only  to  the  slower  of  these  vibrations,  the  heat  rays, 
those  in  the  skin  of  the  frog  may  respond  to  the  whole  series, 
with  what  accompanying  sensation  qualities  we  cannot 
say.  It  is  interesting  to  note  that  Pearse  (566),  working 
with  frogs  and  salamanders,  normal  and  blinded,  finds  that 
red  light,  which  stands  nearest  to  heat  in  vibration  fre- 
quency, is  most  effective  for  the  blinded  animals,  blue  light 
for  the  normal  ones.  In  the  young  of  frogs  and  salaman- 
ders it  has  been  shown  that  the  skin  nerves  are  the  source 
of  dermal  reactions  to  light. 

The  frog's  eye  is  sensitive  to  light  rays  from  all  the 
spectral  regions  visible  to  man ;  the  distribution  of  bright- 
nesses in  the  spectrum  is  like  that  of  normal  human  vision, 
and  the  dark-adapted  eye  shows  a  shift  of  the  brightness 
values  to  correspond  with  those  of  the  dark-adapted  hu- 
man eye.  One  method  by  which  these  results  were  ob- 
tained was  that  of  testing  the  electric  effect  (action  cur- 
rents) of  stimulating  frogs'  eyes  with  light  of  different  colors  : 
the  maximum  effect  for  the  light-adapted  eye  was  in  the 
yellow  green,  that  for  the  dark-adapted  eye  was  in  the  yel- 
low (324  a).  Another  method  was  to  illuminate  food  with 
light  of  different  colors  and  to  observe  in  what  lights  it 
was  most  readily  seized.  From  the  results  Hess  (305) 
concludes  that  amphibian  vision  is  qualitatively  like  that 


Sensory  Discrimination:   Vision  165 

of  man.  Babak  (n)  has  studied  the  effect  of  different 
colors  on  the  frog's  breathing ;  its  rate  and  the  movements 
involved  in  ''throat"  and  "lung"  breathing.  The  fore- 
brains  of  the  animals  had  been  removed,  a  proceeding 
which  makes  the  breathing  of  the  resting  animal  more 
regular.  He  found  that  each  color  produced  a  breathing 
curve  of  a  certain  specific  pattern,  and  concluded  that 
the  colors  have  specific  effects  on  the  eye  independent 
of  their  intensity. 

The  results  with  turtles,  which  are  reptiles,  correspond 
to  those  for  amphibians,  except  that  Hess  (305)  finds  the 
spectrum  shortened  at  the  violet  end ;  that  is,  the  turtle 
does  not  see  beyond  the  blue.  The  method  used  was  that 
of  illuminating  food  with  differently  colored  lights.  Hess 
explains  this  shortening  of  the  spectrum  by  the  fact  that 
in  the  turtle  eyes,  as  in  those  of  all  birds,  a  few  fishes,  and 
Ornithorhyncus,  there  are  attached  to  the  ends  of  the 
cones  transparent  colored  globules  like  little  drops  of  oil. 
They  are  in  the  turtle  mostly  red  and  orange,  and  would 
act,  Hess  thinks,  like  spectacles  of  colored  glass  to  cut  off 
the  blue  and  violet  rays.  The  fact  that  adaptation  to 
darkness  apparently  occurs  in  the  turtle  is  of  interest 
because  its  retina  is  lacking  in  rods.  The  rods,  then,  can- 
not be,  as  they  have  sometimes  been  supposed,  essential 
to  the  process  of  darkness  adaptation. 

§  43.   The  Problem  of  Visual  Qualities:   Birds 

Many  experiments  have  been  made  on  color  discrimina- 
tion in  birds.  Most  of  the  older  ones  were  conducted  by 
the  method  of  training  the  birds  to  choose  between  dif- 
ferently colored  papers  (611),  or  between  compartments 
illuminated  through  differently  colored  glass  (647).  These 


1 66  The  Animal  Mind 

experiments  made  practically  no  attempt  to  guard  against 
the  possibility  that  the  birds  were  reacting  to  differences  in 
the  brightness  of  the  lights.  Another  method  used  by 
Katz  and  Revesz  (395)  was  that  of  scattering  grain  on 
grounds  of  different  colors  and  noting  how  often  the  grain 
was  picked  from  the  several  grounds.  The  positive  result 
of  this  research  was  that  while  fowls  with  light-adapted 
eyes  pecked  equally  often  at  grains  on  yellow,  green,  red, 
and  violet  grounds,  those  with  dark-adapted  eyes  never 
pecked  at  grain  on  the  red  ground.  This  indicates  a  pro- 
cess of  darkness  adaptation  like  that  in  the  human  eye, 
which  sees  red  as  very  dark  in  faint  light.  In  a  later  inves- 
tigation the  same  workers  tried  scattering  red,  blue,  and 
green  grains  of  various  saturations,  mixed  with  grains 
stained  four  different  shades  of  gray.  All  the  grains  were 
stuck  fast  to  the  ground  except  those  of  a  particular  color. 
The  fowls  showed  an  ability  to  discriminate  which  was 
about  equal  to  that  of  the  normal  human  being.  It  can- 
not be  said,  however,  that  these  experiments  satisfactorily 
eliminated  the  brightness  error,  since  so  few  shades  of 
gray  were  used. 

The  effect  of  different  colored  rays  on  the  pupillar  re- 
flex of  birds  was  studied  by  Hess  (303).  For  day  birds, 
he  found  that  the  maximal  effect  was  produced  by  the 
yellow  rays ;  for  owls,  by  the  yellow-green.  That  is,  the  day 
birds  showed  the  brightness  distribution  characteristic  of  the 
light-adapted  human  eye  with  color  vision ;  the  night  birds 
the  distribution  of  total  color  blindness,  or  darkness  adap- 
tation. By  his  method  of  observing  under  what  illumina- 
tion the  animals  could  find  food,  Hess  obtained  results  lead- 
ing him  to  conclude  that  day  birds  have  a  spectrum  short- 
ened in  the  violet  end,  a  fact  which  he  ascribes  to  the  effect 
of  the  oil  globules  in  the  retina ;  and  that  the  spectrum  for 


Sensory  Discrimination:   Vision  167 

owls  is  somewhat  longer.  Watson  (773),  on  the  other 
hand,  working  with  a  more  exact  apparatus,  concludes  that 
the  spectrum  is  visible  to  the  chick  and  the  homing  pigeon 
within  the  same  limits  as  to  man.  A  study  of  the  electric 
currents  generated  by  the  action  of  light  on  the  eyes  of 
day  and  night  birds  gives  evidence  confirming  the  hypothe- 
sis that  the  latter  are  color-blind :  in  the  day  birds,  each 
color  gives  a  characteristic  deflection  of  the  galvanometer, 
not  due  to  its  intensity,  while  no  such  differences  appear  for 
the  eyes  of  night  birds  (403).  Breed  (101,  102),  using 
colored  screens  through  which  the  light  was  passed,  and 
offering  a  choice  of  passages  differently  illuminated,  ob- 
tained evidence  of  color  discrimination  in  the  chick.  The 
preference  of  the  chicks  for  one  color  rather  than  another 
appeared  to  depend  on  the  relative  brightness  of  the 
colors,  since  it  could  be  reversed  when  their  brightnesses 
were  sufficiently  altered.  When  a  blue  and  red  were  found 
between  which  the  chick  showed  no  preference,  this  was 
taken  as  an  indication  that  they  looked  equally  bright  to 
the  chick.  The  bird  could,  however,  be  trained  to  choose 
one  of  these  two  colors ;  hence  the  conclusion  was  reached 
that  it  could  probably  react  to  a  difference  in  color  and 
not  merely  to  one  in  brightness.  The  evidence  for  color 
vision  in  birds  has  lately  been  made  practically  conclusive 
by  the  careful  experiments  of  Lashley  (413)  on  the  domestic 
fowl.  He  used  spectral  light  whose  intensity  was  accu- 
rately controlled.  The  ability  of  the  fowl  to  distinguish 
red  and  green  was  demonstrated  under  the  following  con- 
ditions, which  ruled  out  the  possibility  of  discriminating 
by  brightness  differences,  (i)  Each  of  the  lights  was  al- 
ternately reduced  to  threshold  intensity,  while  the  other 
remained  at  full  intensity.  (2)  White  light  of  a  constant 
intensity  was  substituted  for  each  colored  light  in  turn. 


1 68  The  Animal  Mind 

(3)  Each  light  was  in  turn  exposed  alone,  one  passage  being 
left  dark. 

Indications  of  the  presence  of  darkness  adaptation  in 
the  chick  appeared  from  the  facts  that  light  adapted  chicks 
chose  red  and  yellow  rather  than  green,  while  for  dark 
adapted  chicks  the  preference  was  reversed. 

Rouse  observed  that  differently  colored  lights  had  on 
the  average  different  effects  in  quickening  the  rate  of 
breathing  in  the  pigeon;  the  strongest  effect  being  pro- 
duced by  blue,  the  weakest  by  red  (648). 

§  44.   The  Problem  of  Visual  Qualities:  Mammals 

The  earlier  experiments  on  the  visual  discriminations  of 
mammals,  like  those  with  other  animals,  failed  to  reckon 
adequately  with  the  brightness  error,  the  possibility  that 
discriminations  between  colors  are  made  as  a  color-blind 
human  being  would  make  them,  the  colors  being  seen  as 
different  shades  of  gray  (138,  140).  The  first  method, 
as  we'  have  seen,  which  suggested  itself  as  a  means  of 
eliminating  this  error  was  that  of  showing  that  an  animal 
could,  or  could  not,  distinguish  a  color  from  the  gray 
which  a  light-adapted  human  being  would  see  in  its 
place.  Such  a  gray  can  be  determined  by  the  so-called 
" flicker  method."  If  a  disk  be  made  of  a  colored  and 
a  gray  paper,  when  it  is  rotated  a  little  too  slowly  to 
give  a  smooth  mixture,  the  peculiar  appearance  of  "  flick- 
ering" will  be  observed  if  the  color  and  the  gray  are  not  of 
equal  brightness,  but  will  disappear  when  a  gray  equally 
bright  with  the  color  is  selected.  The  determination  of 
this  equivalent,  however,  has  really  no  bearing  on  the 
problem  of  color  vision  in  animals.  If  they  are  color-blind, 
their  difficulty  would  more  probably  lie  in  distinguishing 


Sensory  Discrimination:   Vision  169 

between  a  color  and  its  brightness  equivalent  for  the  color- 
blind or  dark-adapted  human  eye ;  and  quite  possibly  the 
brightness  which  they  see  instead  of  color  may  be  unlike 
the  brightness  value  of  that  color  to  either  the  light-adapted 
or  the  dark-adapted  human  eye. 

Kinnaman's  (401)  color  tests  on  monkeys,  from  which 
he  concluded  that  they  possess  color  vision,  employed  only 
the  older  methods  of  getting  rid  of  the  brightness  error : 
the  monkeys,  which  had  learned  to  identify  a  vessel  covered 
with  a  particular  colored  paper  as  containing  food,  were 
shown  to  be  unequal  to  the  discrimination  between  gray 
papers  whose  brightnesses  were  to  the  human  eye  the  same 
as  those  of  the  colors.  It  was  also  shown  that  a  colored 
glass  could  be  picked  out  many  times  from  among  three 
others  covered  with  gray  paper  of  the  same  brightness  as 
the  color,  to  human  vision.  In  Cole's  (134)  demonstration 
that  raccoons  can  distinguish  colors,  the  colors  used  were 
equated  in  brightness  for  the  human  eye  by  the  flicker 
method. 

The  experiments  of  Yerkes  on  the  dancing  mouse  (820) 
brought  into  clear  relief  the  danger  of  trying  to  eliminate 
the  brightness  error  by  the  use  of  grays  equal  in  brightness 
with  the  colors  to  the  human  eye.  His  method  consisted 
in  teaching  the  animals  to  associate  one  of  two  differently 
illuminated  compartments  with  an  electric  shock.  The 
intensity  of  the  illuminations  could  be  regulated  by  varying 
the  distance  of  the  lights  from  them.  When  only  white 
lights  were  used,  Weber's  Law  was  found  to  hold  for  the 
one  mouse  tested :  the  animal  could  distinguish  a  differ- 
ence in  the  brightness  of  the  compartments  amounting  to 
about  one-tenth  of  their  absolute  brightness,  within  cer- 
tain limits  of  absolute  brightnesses.  Light  blue  and 
orange,  green  and  red,  violet  and  red,  were  discriminated 


170  The  Animal  Mind 

even  when  their  brightnesses  were  considerably  varied. 
Yet  the  probability  appeared  that  these  discriminations 
were  based  merely  on  brightness  differences,  for  after  a 
mouse  had  learned  to  choose  green  rather  than  red,  when  it 
was  offered  a  choice  between  light  and  darkness,  it  uni- 
formly preferred  the  former,  although  untrained  mice 
showed  no  such  preference.  Apparently,  then,  the  green 
had  been  previously  discriminated  simply  as  the  lighter  of 
the  two  impressions,  and  to  the  eye  of  the  mouse,  as  to  that 
of  the  color-blind  human  being,  red  looks  an  extremely 
dark  gray. 

In  some  experiments  of  the  writer's  (756)  on  the  rabbit, 
the  method  was  used  of  presenting  a  color  with  various 
grays,  in  successive  experimental  series,  and  rinding  whether 
or  not  there  existed  a  gray  with  which  the  color  was  confused. 
This  is  the  only  adequate  way  of  dealing  with  the  bright- 
ness error.  We  found  that  while  the  rabbit  could  be  taught, 
by  rewarding  it  with  food  for  right  choices,  to  distinguish 
a  standard  red  paper  from  a  number  of  different  gray 
papers,  it  invariably  failed  when  a  very  dark  gray,  almost 
black,  was  presented  with  the  red.  Two  objections  which 
have  been  urged  against  the  use  of  colored  papers  were 
met  in  these  experiments.  In  the  first  place,  it  is  argued 
that  papers  of  different  colors  may  differ  in  surface  texture : 
the  possibility  that  our  rabbits  reacted  to  this  clew  we 
eliminated  by  occasionally  substituting  red  and  gray  vel- 
vets for  our  red  and  gray  papers,  a  change  that  did  not  at 
all  affect  the  rabbits.  Secondly,  it  has  been  urged  that 
when  colored  papers  are  pasted  on  cards,  they  are  apt  to 
show  wrinkles  that  might  identify  them :  this  we  obvi- 
ated by  pinning  on  our  papers  afresh  in  successive  experi- 
ments. 

Obviously,  however,  since  colored  papers  do  not  give 


Sensory  Discrimination:   Vision  171 

pure  colors ;  since,  that  is,  they  reflect  to  the  eye  light  of 
wave-lengths  besides  that  of  their  predominant  color,  the 
use  of  pure  spectral  light  is  to  be  preferred.  The  ap- 
paratus by  which  such  light  can  be  used  with  its  intensity 
accurately  controlled  is  very  elaborate,  and  was  devised 
by  Yerkes  and  Watson  (831).  Using  this  apparatus,  the 
Watsons  (772)  found  that  rats  and  rabbits  failed  to 
distinguish  between  red  and  darkness.  A  similar  indica- 
tion had  been  previously  obtained  by  Waugh  (775)  on  the 
gray  mouse ;  he  found  that  red  niters  and  pigments  could 
be  distinguished  from  gray  when  the  two  were  equal  in 
brightness  to  the  human  eye,  but  that  the  discrimination 
tended  to  disappear  when  the  red  was  made  lighter,  and 
to  improve  when  it  was  made  darker. 

The  dog  and  cat  also  suffer  under  the  imputation  of  color- 
blindness. Tests  by  Pawlow's  method  on  the  dog  failed 
to  indicate  that  it  can  react  to  color  differences  as  such 
(830) .  Smith  (687),  it  is  true,  working  with  colored  papers, 
argues  in  favor  of  the  dog's  color  vision  from  the  fact  that 
the  dogs  showed  some  evidence  of  learning  to  distinguish 
the  colors  from  all  the  grays  used.  De  Voss  and  Ganson 
(184)  found  that  none  of  the  six  colored  papers  they  used 
could  be  discriminated  by  cats  from  all  the  shades  of  gray 
in  their  series :  each  color  was  confused  with  some  partic- 
ular gray.  Even  the  monkey  is  suspected  of  color-blind- 
ness: Watson  (768)  reports  as  the  chief  result  of  his 
experiments  with  this  animal  that  red  has  little  or  no 
stimulating  power  upon  it. 


CHAPTER  VIII 

SPATIALLY   DETERMINED    REACTIONS    AND    SPACE 
PERCEPTION 

§  45.  Classes  of  spatially  determined  reactions 

MODIFICATION  of  the  behavior  of  animals  with  reference 
to  the  spatial  characteristics  of  the  forces  acting  upon  them 
appears  at  the  very  beginning  of  the  scale  of  animal  life,  and 
throughout  is  quite  as  important  as  modification  with  refer- 
ence to  the  kind  or  quality  of  such  forces.  It  assumes  a 
number  of  distinct  forms.  Some  of  these  suggest  to  us, 
interpreting  them  as  we  must  on  the  basis  of  our  own 
experience,  no  conscious  aspect  at  all;  they  seem  rather 
mechanical  effects  upon  a  passive  organism.  In  other 
cases,  it  appears  possible  that  the  mental  process  which  we 
know  as  space  perception,  involving  the  simultaneous 
awareness  of  a  number  of  sensations  consciously  referred 
to  different  points  in  space,  may  accompany  the  reaction  of 
an  animal  with  reference  to  the  spatial  relations  of  its 
environment.  And  sometimes  we  can  only  say  that  differ- 
ences in  the  space  characteristics  of  a  stimulus  may  modify 
the  accompanying  sensation  in  some  manner  which  yet 
apparently  does  not  involve  space  perception  as  we  know  it. 

Our  task  in  the  following  pages  will  then  be  to  examine 
the  different  ways  in  which  animal  behavior  is  adapted  to 
the  spatial  characteristics  of  stimuli,  and  to  ask  which  of 
these  suggest  as  their  conscious  accompaniment  some 
form  of  space  perception.  A  classification  of  spatially 

172 


Spatially  Determined  Reactions  173 

determined  responses  that  is  not,  indeed,  ideally  satis- 
factory, but  may  serve  our  purpose,  divides  them  into  five 
groups : — 

1 .  Reactions  adapted  to  the  position  of  a  single  stimulus 
acting  at  a  definite  point  on  the  body.    , 

2.  Reactions  to  a  continuous  stimulus,  which  involve  the 
assumption  of  a  certain  position  of  the  whole  body  with 
reference  to  the  stimulus :  orienting  reactions. 

3.  Reactions  to  a  stimulus  that  moves,  i.e.,  that  affects 
several  neighboring  points  on  the  body  successively. 

4.  Reactions  adapted  to  the  relative  position  of  several 
stimuli  acting  simultaneously. 

5.  Reactions  adapted  to  the  distance  of  an  object  from 
the  body. 

These  forms  of  behavior  will  be  successively  discussed. 

§  46.  Class  I :  Reactions  to  a  Single  Localized  Stimulus 

Responses  to  stimulation  that  are  adapted  to  the  point 
of  application  of  the  stimulus  are  to  be  found  among  very 
simple  animals.  They  may  be  subdivided  into  three 
groups:  first,  cases  where  the  part  of  the  animal  that 
reacts  is  the  part  directly  affected  by  the  stimulus ;  second, 
cases  where  the  whole  animal  reacts  by  a  movement  in  the 
appropriate  direction;  and  third,  cases  where  a  part  of 
the  body  not  directly  affected  by  the  stimulus  moves  toward 
the  point  stimulated. 

i.  Amoeba  furnishes  an  example  of  the  first  class.  Its 
negative  reaction  occurs  by  the  checking  of  protoplasmic 
flow  at  the  point  where  a  strong  mechanical  stimulus  affects 
the  body ;  its  positive  reaction  by  a  flowing  forward  of  the 
protoplasm  at  the  point  where  a  weak  stimulus  acts,  and  its 
food-taking  reaction  by  an  enveloping  flow  on  both  sides  of 


174  The  Animal  Mind 

the  point  stimulated.  This  would  seem  to  be  the  most 
primitive  way  of  adapting  response  to  the  location  of  a 
stimulus :  the  effect  is  produced  just  where  the  force 
acts,  as  it  might  be  upon  a  piece  of  inanimate  matter.  In 
no  animal  with  a  nervous  system,  probably,  is  the  process 
quite  so  simple.  The  beH  of  the  jellyfish  contracts  at  the 
point  where  a  stimulus,  mechanical  or  photic,  is  applied; 
yet  although  these  responses  are  made  when  the  nervous 
system  is  thrown  out  of  function,  they  occur  more  slowly, 
and  in  the  normal  animal  the  nervous  tissue  is  probably 
involved,  while,  of  course,  a  long  conduction  pathway  is 
traversed  when,  to  use  a  familiar  illustration,  the  baby  pulls 
back  its  hand  from  the  candle  flame. 

2.  Paramecium  and  other  infusoria,  planarians,  the 
earthworm,  and  various  other  animals  give  us  illustrations 
of  movements  of  the  entire  body  differing  according  to  the 
point  affected  by  a  single  stimulus.  If  the  front  half  of 
Paramecium  be  touched,  the  animal  gives  the  typical  avoid- 
ing reaction  of  darting  backward  and  turning  to  one  side ; 
if  the  hinder  end  be  touched,  it  moves  forward  (378,  p.  59). 
On  the  other  hand,  it  makes  no  difference  in  its  reactions 
to  stimuli  affecting  either  side  of  the  body ;  the  turning  is 
always  to  the  aboral  side  even  when  the  stimulus  comes 
from  that  direction  (378,  p.  52).  If  strong  mechanical 
stimulation  be  applied  to  the  head  end  of  a  planarian,  there 
is  a  response  which  seems  to  belong  under  type  (i) :  the 
head  is  turned  away  from  the  stimulus.  If  the  hinder  region 
is  touched,  strong  forward  crawling  movements  of  the  body 
are  produced.  The  positive  reaction  in  the  planarian, 
turning  the  head  toward  the  stimulus,  also  suggests  type  (i), 
but  in  reality  it  has  been  shown  by  Pearl  to  be  a  far  more 
complex  affair  than  the  mere  flow  of  protoplasm  at  the 
stimulated  point,  and  to  involve  the  contraction  of  several 


Spatially  Determined  Reactions  175 

sets  of  muscles  (561).  The  earthworm  creeps  backward 
if  the  front  half  of  the  body  is  affected,  turns  away  from 
a  stimulus  applied  to  the  side  of  the  anterior  end,  and  creeps 
forward  if  the  stimulus  affects  the  posterior  half  of  the  body 
(377).  In  general,  a  reaction  of  type  (2)  rather  than  type 
(i)  will  occur  in  proportion  to  the  degree  in  which  an 
organism's  movements  are  coordinated  and  it  tends  to 
act  as  a  whole. 

3.  One  of  the  prettiest  examples  of  the  most  highly  co- 
ordinated form  of  response  to  a  single  localized  stimulus; 
namely,  movement  of  some  other  part  of  the  body  toward 
the  point  affected,  is  to  be  found  in  the  swinging  over  of  the 
jellyfish's  manubrium  toward  the  spot  on  the  bell  touched 
by  food.  "In  the  typical  feeding  reaction,"  says  Yerkes, 
"the  manubrium  bends  toward  the  food.  If  during  such 
a  movement  the  piece  of  food  be  moved  to  the  opposite 
side  of  the  bell,  the  manubrium,  too,  in  a  few  seconds  will 
bend  in  the  opposite  direction,  that  is,  again  toward  the 
food"  (802).  The  sea  urchin  responds  to  mechanical 
stimulation  by  moving  the  spines  toward  the  place  stimu- 
lated (735).  In  the  higher  animals  this  form  of  reaction  has 
largely  superseded  other  methods  of  adapting  behavior  to 
a  stimulus  acting  at  a  definite  point.  Where  grasping 
appendages  exist,  the  obvious  device  is  to  move  them  toward 
the  point  of  stimulation  in  order  either  to  seize  or  to  remove 
the  object.  This  involves  not  merely  that  the  effects  of  the 
stimulus  shall  diffuse  so  as  to  involve  general  locomotor 
movements,  but  that  the  effect  shall  be  exerted  very  defi- 
nitely upon  a  particular  set  of  muscles  in  a  particular  way. 
The  "scratch-reflex"  of  mammals,  and  the  reaction  whereby 
a  frog  rubs  its  hind  leg  on  the  spot  of  skin  affected  by  a 
drop  of  acid,  are  further  examples. 

What  can  we  say  regarding  the  conscious  accompani- 


176  The  Animal  Mind 

ment  of  the  reactions  described  under  these  three  heads? 
When  a  stimulus  applied  at  point  a  brings  about  a  reaction 
different  from  that  produced  by  precisely  the  same  stimulus 
acting  on  point  b,  are  the  accompanying  sensations  differ- 
ent, supposing  the  animal  concerned  to  be  conscious?  If 
they  are,  the  difference  must  be  what  has  been  called  a 
difference  in  local  sign.  There  is  certainly  no  evidence  that 
space  perception  is  concerned.  Space  perception  in  our 
own  experience  always  involves  the  simultaneous  awareness 
of  several  stimuli.  But  where  a  single  stimulus  only  is 
operative,  the  fact  that  reaction  to  it  is  modified  by  its 
location  cannot  mean  that  the  relations  of  that  location 
to  the  location  of  other  stimuli  are  perceived.  The  truth 
is  that  space  perception  is  so  constant  a  factor  in  our  own 
experience  that  we  cannot  imagine  how  a  single  sensation 
can  be  modified  in  connection  with  change  of  place  of  the 
stimulus,  where  space  perception  does  not  exist.  A  touch 
at  any  point  on  the  skin  of  a  human  being  is  referred  to  a 
definite  point  in  a  constricted  space,  tactile  and  visual; 
it  is  given  its  proper  place  in  a  complex  of  sensations. 
What  modification  of  it  would  correspond  to  its  location  if 
it  stood  alone  in  consciousness,  we  cannot  now  conceive. 

§  47.   Class  II:    Orienting  Reactions;    Possible  Modes  of 
Producing  Them 

Various  forces,  such  as  gravity,  light,  electricity,  centrif- 
ugal force,  currents  of  water  and  air,  are  all  influences 
causing  certain  organisms  to  bring  their  bodies  into  a  defi- 
nite position.  Such  reactions,  involving  the  direction  of 
the  whole  body  with  reference  to  a  continuous  force  acting 
upon  it,  are  known  as  reactions  of  orientation.  There  are 
various  ways  in  which  they  might  conceivably  take  place. 


Spatially  Determined  Reactions  177 

(a)  They   might    be   due    to    the    "pull"    of    a   force 
upon  the  passive  body  of  an  animal.     In  the   case  of 
gravity  or  of  a  current  of  wind  or  water,  if  one  part 
of    the   body   were  heavier   or   offered ,  more   surface   to 
the  force,  the  position  assumed  could  be  explained  with- 
out  supposing   any   activity   on    the   animal's  part.     In 
such  a  case  there  would  be  no  reason  for  thinking  of  the 
reaction  as  conscious. 

(b)  The  response  might  be  due  to  the  effect  of  a  force 
acting  unevenly  upon  the  two  sides  of  the  body,  and  thereby 
unevenly  affecting  the  motor  apparatus  on  the  two  sides, 
thus  causing  the  animal  to  turn  until  the  forces  acting  upon 
symmetrical  points  were  balanced.     This,  although  involv- 
ing activity  on  the  animal's  part,  would  not,  if  the  force 
acted  directly  on  the  muscles,  suggest  any  conscious  ac- 
companiment.    If  it  acted  through  symmetrically  placed 
sense  organs,  awareness  of  the  direction  from  which  the 
force  operated  might  be  present. 

(c)  The  orientation  might   take  place  by  a  negative 
reaction  on  the  animal's  part  to  a  definite  stimulus  given 
when  the  animal  was  in  any  other  than  the  final,  oriented 
position.     If  gravity  were  the  force  in  question,  the  stimu- 
lus might  be  the  pressure  exerted  within  the  body  by 
particles  of  different  density  or  by  the  fluid  or  mineral 
bodies  in  a  statocyst  organ.     If  the  stimulus  were  light, 
the  organism  might  be  oriented  by  giving  the  negative 
reaction  when  its  head  entered  a  region  either  brighter  or 
darker  than  the  optimum  illumination.     In  such  cases, 
where  the  ordinary  negative  reaction  is  the  only  one  in- 
volved, there  is  no  reason  to  suppose  the  occurrence  of  any 
conscious   accompaniment,    other   than   the   possible   un- 
pleasantness connected  with  that  reaction. 

(d)  Orientation  to  gravity  might  occur  through  a  special- 


178  The  Animal  Mind 

ized  " righting"  reaction,  given  in  response  either  to  a  stimu- 
lus within,  say,  a  statolith  organ,  or,  as  in  the  planarian,  to 
the  absence  of  accustomed  contact  stimulation  on  one  sur- 
face of  the  body.  The  reaction  in  these  cases  being  a 
specialized  one,  it  is  possible  that  a  peculiar  sensation 
quality  might  be  involved. 

(e)  Orientation  might  take  place  through  a  movement 
occurring  when  the  position  of  several  stimuli  perceived 
simultaneously  was  disturbed,  and  tending  to  restore  them 
to  their  original  position.  This  is  the  principle  involved, 
as  we  shall  see,  in  explaining  the  rheotropism  or  current 
orientation  of  fishes,  and  the  anemotropism,  or  orientation 
to  air  currents,  of  insects,  as  due  to  an  instinct  to  keep  the 
visual  surroundings  the  same.  And  this  form  of  orienta- 
tion alone  suggests  a  true  space  perception  as  its  conscious 
accompaniment. 

Such  being  the  conceivable  ways  in  which  orientation 
may  be  brought  about,  what  are  the  observed  facts  ?  They 
may  be  considered  under  the  heads  of  orientation  to  gravity, 
to  light,  and  to  other  forces. 

§  48.  Orientation  to  Gravity:  Protozoa 

To  this  form  of  reaction  the  term  "geotropism"  or  "geo- 
taxis"  has  been  applied.  In  various  Protozoa  negative  ge- 
otropism,  or  a  tendency  to  rise  against  the  pull  of  gravity, 
has  been  observed :  first  by  Schwartz  in  two  single-celled 
organisms  frequently  classified  as  plants,  Euglena  and 
Chlamydomonas  (667) ;  and  eight  years  later  by  Ader- 
hold,  who  suggested,  without  accepting  it,  the  theory  that 
the  orientation  may  be  due  simply  to  the  greater  weight  of 
one  end  of  the  organism's  body  (2).  This  view  was  main- 
tained by  Verworn :  the  action  of  gravity,  he  urged,  must 


Spatially  Determined  Reactions  179 

be  purely  passive.  It  cannot  operate  as  a  stimulus  to 
active  response  on  the  animal's  part,  for  a  stimulus  is 
always  a  change  in  environment,  and  gravity  is  a  constant 
force  (742).  This  ignores  the  fact  that  the  animal's  rela- 
tions to  gravity  may  change  though  gravity  does  not,  and 
also  the  fact  that  the  continuous  action  of  light  is  a  stimulus. 
According  to  Verworn's  theory,  the  geotropic  orientation 
of  a  single-celled  organism  takes  place  through  a  series  of 
"little  falls"  whereby  the  heavier  end  is  directed  downward. 
Massart  opposed  this  view  on  the  basis  of  observations 
which  showed  that  the  actual  movements  of  the  organisms 
did  not  correspond  to  it,  but  were  the  result  of  active 
orientation.  If  response  to  gravity  is  passive,  then  dead 
animals  should  fall  through  the  water  in  the  same  position 
as  that  assumed  by  living  animals  when  oriented  to  gravity. 
Massart  experimented  with  various  Protozoa  by  killing 
them  and  studying  their  positions  in  sinking,  which  he 
found  not  always  the  same  as  the  attitudes  assumed  in 
response  to  gravity  (461).  There  is  always  the  possibility, 
however,  that  the  methods  employed  to  kill  may  change  the 
specific  gravity  of  some  part  of  the  body.  Jensen  offered 
the  theory  that  reaction  to  gravity  may  be  due  to'  the  differ- 
ence in  the  water  pressure  on  the  two  ends  of  the  animal. 
He  asserted  that  when  the  air  pressure  on  the  water  was 
reduced  by  exhausting  the  air  above,  there  was  an  increase 
in  the  geotropism,  indicating  a  relative  rather  than  an 
absolute  sensibility  to  pressure  (382),  but  Lyon  points  out 
that  this  process  may  affect  the  animals  in  various  other 
ways  besides  altering  the  air  pressure.  Increasing  the  air 
pressure,  or  protecting  the  surface  with  oil,  has  no  effect 
upon  geotropism,  Lyon  finds,  and  he  urges  that  Jensen's 
theory  requires  enormous  sensibility  to  pressure  differences 
on  the  organism's  part,  as  great  as  that  needed  by  a  human 


180  The  Animal  Mind 

being  to  note  the  difference  between  the  air  pressure  on 
the  head  and  that  on  the  feet  (449).  Another  suggestion 
was  offered  by  Davenport  (175),  namely,  that  negatively 
geotropic  organisms  swim  in  the  direction  where  the 
greatest  resistance  to  their  progress  is  offered.  This  is 
like  one  theory  put  forward  to  explain  rheotropism,  or  the 
tendency  of  animals  to  swim  against  currents,  and  anemot- 
ropism,  or  the  "head  against  wind"  movement  of  insects; 
and  as  Radl  (622)  first  and  Lyon  (448)  afterward  pointed 
out,  it  assumes  the  fact  to  be  explained,  for  only  if  an 
animal  actively  opposes  a  force,  will  that  force  exert  more 
pressure  at  one  point  of  its  body  than  at  another.  The 
theory  cannot  explain  why  an  animal  at  rest  should  be 
oriented.  Another  argument  that  tells  against  it  is  offered 
by  experiments  showing  that  animals  placed  in  solu- 
tions of  the  same  density  as  their  own  bodies,  in  which, 
therefore,  they  have  no  weight,  still  display  negative 
geotropism,  and  that  the  direction  of  the  response  is 
not  reversed  when  the  fluid  is  made  heavier  than  the 
animals  (449).  Lyon's  own  theory,  accepted  by  Jennings, 
is  that  the  stimulus  for  geotropism  is  furnished  by  the 
action  of  gravity  within  the  body  of  the  organism,  upon 
substances  of  different  weight  which  exert  varying  pressures 
and  take  up  different  positions  according  to  the  position  of 
the  body  (449). 

Harper  (289)  in  1911  revived  the  mechanical  theory  of  the 
geotropism  of  Paramecium.  He  argued  that  an  animal 
which,  like  this  protozoon,  moved  in  a  spiral  could  hardly 
use  the  changes  of  position  of  internal  particles  as  effective 
stimuli.  The  reaction  of  Paramecium  can  be  altered  by 
altering  the  specific  gravity  of  its  body,  as  by  causing  it  to 
absorb  particles  of  iron  or  paraffin.  When  it  has  ingested 
iron,  its  responses  are  modified  by  the  neighborhood  of  a 


Spatially  Determined  Reactions  181 

magnet  (290).  Wager  (751)  maintains  that  geotropism.  in 
Euglena  also  is  purely  passive,  due  to  the  fact  that  the 
hinder  end  of  the  animal  is  the  heavier.  Kanda  (389) 
has  recently  championed  the  "statocyst"  theory  of  Lyon, 
as  against  the  mechanical  theory,  using  Lyon's  argument 
that  when  Paramecia  are  rapidly  rotated  in  an  apparatus 
called  a  "  centrifuge, "  their  front  ends  are  directed  outward 
by  centrifugal  force  and  therefore  must  be  heavier,  instead 
of  lighter,  as  the  mechanical  theory  would  require.  Harper 
had  previously  attempted  to  meet  this  objection  by  re- 
garding such  a  position  on  the  part  of  the  centrifuged 
animals  as  due  not  to  centrifugal  force  but  to  compensatory 
movements  made  actively  by  the  animal.  Both  the 
mechanical  theory  and  the  "statocyst"  theory,  then,  seem 
to  be  still  on  the  field. 

It  has  been  shown  that  the  reactions  of  Paramecium  to 
gravity  are  modified  by  a  variety  of  conditions.  Negative 
geotropism,  in  a  sense  its  normal  condition,  is  favored  by 
plentiful  food  supply  and  by  an  increase  in  temperature 
within  certain  limits ;  positive  geotropism,  movement  down- 
ward, may  be  brought  about  temporarily  by  mechanical 
shock,  by  salts  and  alkalies,  by  temperature  changes  (503, 
689) ,  to  which,  however,  the  animals  may  adapt  themselves ; 
with  less  constancy  by  increase  in  the  density  of  the  fluid 
containing  them,  and  with  lasting  effect  by  lack  of  food. 
It  has  been  suggested  that  the  downward  movement  under 
these  circumstances  is  protective,  since  it  shields  the  animals 
from  surface  agitation  of  the  water,  from  surface  ice,  and 
from  failure  of  the  surface  food  supply  (500).  We  shall 
see  that  similar  conditions  often  change  the  direction  of  an 
animal's  response  to  light. 


1 82  The  Animal  Mind 

§  49.  Orientation  to  Gravity:  Codenterates 

Among  the  ccelenterates,  geotropisin  is  shown  by  certain 
hydroids,  whose  stems  have  a  tendency  to  curve  upward 
and  their  "roots"  a  tendency  to  grow  vertically  downward 
when  the  animals  are  placed  in  a  horizontal  position  (714). 
The  sea-anemone  Cerianthus,  whose  normal  position  is  head 
upward,  will  right  itself  if  placed  in  any  other  position, 
though  the  righting  reaction  may  be  inhibited  by  contact 
stimulation  on  the  side  of  the  animal.  It  ordinarily  lives 
with  the  body  enclosed  in  a  tube,  and  when  taken  from  its 
proper  habitat  it  seems  to  "prefer"  a  position,  even  hori- 
zontal, where  the  sides  of  the  body  are  in  contact  with  a 
solid,  to  a  vertical  position  with  its  sides  uncovered  (424). 
The  righting  reaction  of  Hydra  is  not  determined  by  gravity 
at  all ;  the  animal  will  take  any  position,  vertical  or  hori- 
zontal, but  "seeks"  always  to  have  its  foot  in  contact  with 
a  solid  (751  a).  Some  actinians  have  shown  an  interesting 
modification  of  gravity  reaction  through  what  we  may  call 
habit.  Six  specimens  of  Actinia  equina  were  selected  that 
had  been  fixed  to  the  rocks  in  an  "upside-down"  position, 
that  is,  with  the  mouth  end  downward ;  and  six  others  that 
had  been  right  side  up.  In  the  first  experiment  all  were 
placed  upside  down ;  the  tendency  to  right  themselves  was 
decidedly  stronger  in  those  which  had  been  previously  erect. 
Similarly,  when  twelve  selected  in  the  same  way  were  all 
placed  right  side  up,  the  ones  that  had  previously  been  in 
the  reversed  position  showed  a  certain  inclination  to 
reassume  it  (258).  On  the  other  hand,  the  orientation  of 
the  polyp  Corymorpha  palma  to  gravity  was  entirely  un- 
affected by  keeping  the  animal  for  a  long  time  in  a  position 
where  it  could  not  right  itself;  it  assumed  the  upright 
position  as  soon  as  it  was  set  free  (714). 


Spatially  Determined  Reactions  183 

It  was  noted  in  the  chapter  on  hearing  that  the  peculiar 
organs  occurring  in  certain  Ccelenterata  and  in  many  other 
animals,  which  were  originally  called  otocysts  because  of 
their  supposed  auditory  function,  have  had  their  name 
changed  to  that  of  statocyst  since  it  has  appeared  that  they 
subserve  chiefly  orientation  to  gravity.  In  jellyfish,  re- 
moval of  these  organs  does  not  seem  to  affect  the  animal's 
power  of  keeping  its  balance;  apparently  equilibrium  is 
maintained  here  by  the  simple  action  of  gravity,  for  dead 
jellyfish  float  in  the  right-side-up  position  (514,  521).  It 
has  been  suggested  that  the  statocyst  organs  are  for  the 
reception  of  stimuli  produced  by  shaking,  to  which  medusae 
are  apparently  sensitive  (521).  Negative  geotropism  exists 
in  Gonionemus,  which  swims  to  the  surface  of  the  water 
when  disturbed  (825).  In  ctenophors,  the  statocyst  organ, 
which  is  usually  at  one  pole  of  the  body,  has  been  found  to 
function  as  an  organ  for  the  maintenance  of  equilibrium 

(741). 

§  50.   Orientation  to  Gravity:  Planarians 

A  good  example  of  a  specially  developed  reaction  having 
for  its  result  the  "righting"  of  an  animal  in  an  abnormal 
position  is  offered  by  the  behavior  of  a  planarian  that  has 
been  turned  over  so  that  its  back  rests  on  the  surface  of 
support.  The  reaction  consists  of  a  turning  of  the  body, 
beginning  with  the  head  end,  about  the  long  axis,  so  that 
a  spiral  form  is  assumed.  The  dorsal  surface  of  the  animal 
is  convex,  the  greatest  thickness  of  the  body  being  in  the 
middle  line.  When  the  planarian  lies  on  its  back,  it  thus 
naturally  tips  to  one  side,  like  a  keeled  boat  out  of  water. 
This  side,  being  brought  into  contact  with  a  solid,  gives 
a  reaction  analogous  to  the  negative  one,  that  is,  it  extends 
or  stretches.  Such  a  stretching  of  one  side  when  the 


184  The  Animal  Mind 

planarian  is  right  side  up  would  of  course  produce  a  turn- 
ing in  the  opposite  direction,  a  negative  reaction.  In  this 
case,  however,  the  opposite  side  does  not  contract  to 
allow  of  turning,  but  maintains  the  same  length.  The 
necessary  result  is  that  the  body  is  thrown  into  a  spiral :  as 
soon  as  the  ventral  surface  of  the  head  comes  into  contact 
with  the  solid,  in  consequence  of  the  turning,  the  negative 
reaction  of  that  end  ceases.  Thus  the  righting  is  pro- 
gressively accomplished  (561).  The  whole  response  can 
hardly  be  classed  under  the  head  of  geotropism.  Like 
that  of  Hydra,  it  is  not  made  as  the  result  of  the  pull  of 
gravity,  but  is  a  reaction  to  contact  stimulation;  the 
animal  will  crawl  in  an  upside-down  position  as  readily 
as  any  other  provided  that  the  ventral  surface  and  not  the 
dorsal  is  in  contact  with  a  support. 

§  51.  Orientation  to  Gravity:   Annelids 

Geotropism,  in  the  marine  worm  Conwluta  roscoffensis, 
has  been  found  to  fluctuate  with  the  rise  and  fall  of  the  tides, 
even  when  the  animal  is  removed  to  an  aquarium.  In 
normal  life  the  worms  burrow  in  the  sands  at  rising  water, 
and  come  to  the  surface  when  the  tide  retreats.  Prolonged 
exposure  to  air,  or  increase  in  the  intensity  of  the  light, 
causes  them  to  move  down  the  slope  of  the  shore  to  moist 
places.  These  movements  in  the  normal  environment  are 
represented  by  upward  and  downward  movements  of  the 
animal  when  confined  in  a  glass  tube.  Keeble  and  Gamble 
thought  these  oscillations  in  geotropism  did  not  occur  in 
darkness,  and  that  the  stimulus  bringing  them  about  was 
photic.  When  the  summation  of  light  stimuli  passes  a  cer- 
tain amount,  they  maintained,  positive  geotropism  appears ; 
when  the  after  effect  of  light  stimulation  is  dissipated,  the 


Spatially  Determined  Reactions  185 

negative  phase  recurs  (253) .  Bohn,  however,  finds  that  the 
oscillations  do  persist  in  darkness,  and  that  their  primary 
cause  is  the  mechanical  shock  of  the  waves,  as  is  further 
indicated  by  the  observation  that  shaking  the  tube  will 
cause  the  worms  to  descend  (61).  The  geotropism  of  Con- 
voluta  is  dependent  on  the  statocyst  (253). 


§  52.   Orientation  to  Gravity:  Mollusks 

Among  Mollusks,  the  slug  has  had  its  reactions  to  gravity 
carefully  observed.  When  placed  in  a  horizontal  position 
on  an  inclined  glass  plate,  these  animals  tend  to  turn  either 
upward  or  downward,  moving  either  with  or  against  the 
force  of  gravity.  Davenport  and  Perkins  found  that  the 
same  individuals  differed  at  different  times  in  this  respect,  and 
concluded  that  the  sense  of  the  geotropism  was  determined 
by  obscure  conditions.  They  also  found  that  an  inclination 
of  only  7.5°  on  the  part  of  the  glass  plate,  representing  only 
13°  of  the  full  force  of  gravity,  is  sufficient  to  make  the  slugs 
orient  themselves  with  reference  to  the  pull  of  the  earth, 
though  the  precision  of  such  orientation  increases  as  the 
angle  increases  (175).  Frandsen  thought  it  was  the 
weight  of  the  posterior  part  of  the  body  that  determined 
whether  the  movement  should  be  up  or  down :  that  the 
natural  tendency  of  all  was  to  go  downward,  but  that  in 
some  individuals  the  posterior  part,  which  is  poorly  con- 
trolled, was  heavier  than  the  anterior,  and  pulled  the  animal 
around  head  upward  (236). 

Kanda  (392,  393),  on  the  other  hand,  thinks  that  in 
freshwater  and  marine  snails  the  statoliths  are  the  organs 
determining  orientation  to  gravity,  and  that  it  is  not 
merely  passive:  he  claims  to  have  observed  that  this 
orientation  is  most  marked,  the  less  the  slope  of  the  surface 


1 86  The  Animal  Mind 

on  which  the  animal  crawls.  The  response  of  Physa,  a 
freshwater  snail,  to  gravity  depends  in  an  interesting  way 
on  the  animal's  physiological  condition :  when  the  snail 
is  in  need  of  air  it  is  strongly  negative  in  its  geotropism, 
crawling  upward  towards  the  surface  of  the  water  and  dis- 
regarding all  other  stimuli :  as  soon  as  its  lungs  are  full  of  air 
it  is  no  longer  sensitive  to  gravity  (177).  Buddenbrock 
(107,  108,  109)  and  Baunacke  (29-32)  have  brought  evi- 
dence to  support  the  view  that  the  statocysts  in  many 
mollusks  are  useful  not  so  much  in  securing  orientation  to 
gravity,  which  is  of  little  importance  in  such  slow-moving 
animals,  but  rather  in  enabling  them  to  right  themselves, 
to  direct  their  movements,  and  to  dig  in  the  sand.  The 
statocyst  organs  in  a  cephalopod,  Eledone,  have  been 
shown  to  function  in  maintaining  equilibrium  (249). 

§  53.   Orientation  to  Gravity:  Echinoderms 

Righting  reactions  in  the  starfish  have  been  described  by 
Romanes  (641).  The  tips  of  two  or  three  rays  are  twisted 
around  until  the  suckers  in  the  ventral  side  have  a  firm 
hold  of  the  supporting  surface;  the  twisting  is  then  con- 
tinued, always  in  the  same  direction  on  the  different  rays, 
until  the  whole  body  is  turned.  Jennings  (380)  enumerates 
twelve  different  factors  which  determine  which  particular 
rays  shall  twist  over  and  attach  themselves  first,  but 
Moore  (501)  thinks  that  the  "positive  stereo tropism, " 
that  is,  the  tendency  to  remain  in  contact  with  solids,  of  the 
tube  feet  is  a  sufficient  explanation.  It  is  not  clear  how  a 
tendency  to  remain  in  contact  with  a  mechanical  stimulus 
can  explain  a  tendency  to  seek  such  a  stimulus  when  it  is 
absent,  and  Jennings's  view,  that  the  original  impulse  to 
turn  comes  from  the  general  state  of  unrest  in  which  the 


Spatially  Determined  Reactions  187 

animal  is  thrown  by  its  position,  seems  plausible.  But 
what  is  the  stimulus  inducing  the  unrest?  Not  contact 
of  the  back  with  a  solid  object,  for  a  starfish  is  not  disturbed 
if  its  back  is  touched  when  it  is  crawling  in  the  ordinary 
position;  and  not  merely  having  its  back  directed  down- 
ward, for  it  will  crawl  upside  down  on  the  under  surface 
of  rocks.  Something  abnormal  about  the  stimulation 
of  the  tube  feet  when  they  are  in  contact,  not  with  a 
solid  support,  but  with  the  water  flowing  over  them, 
must  present  the  condition  for  the  internal  state  of 
instability  which  occasions  the  twisting  movements  of 
the  rays. 

The  sea-urchin,  "a  rigid,  non-muscular,  and  globular 
mass,"  as  Romanes  calls  it,  with  relatively  feeble  suckers, 
has  a  much  harder  time  to  right  itself,  and  does  not  succeed 
in  pulling  itself  over  unless  it  is  perfectly  fresh  and  vigorous. 
It  occasionally  rests  for  some  time  when  it  has  reached  a 
position  of  stability  halfway  over,  before  continuing  the 
process  (641). 

Lyon  has  observed  marked  negative  geotropism  in  the 
larvae  of  the  sea  urchin.  He  was  unable  to  test  Davenport's 
theory  of  the  nature  of  the  geo tropic  response  by  putting  the 
animals  in  a  solution  of  the  same  density  as  their  own  bodies, 
for  the  reason  that  such  a  fluid  was  too  dense  and  sticky 
(being  made  of  gum  arabic  and  sea  water)  for  them  to 
swim  in.  That  the  response  was  merely  a  passive  one 
he  thinks  improbable,  because  the  larvae  from  eggs  that 
have  been  rapidly  rotated,  or  "centrifuged,"  as  it  is 
called,  have  all  the  pigment  on  one  side  of  their  bodies 
and  may  therefore  be  supposed  to  have  their  ordinary 
balance  disturbed;  yet  they  rise  to  the  surface  just  like 
the  rest  (450). 


1 88  The  Animal  Mind 

§  54.  Orientation  to  Gravity:    Crustacea 

That  the  statocyst  organs  in  Crustacea  are  probably  con- 
nected with  equilibrium  rather  than  with  hearing  we  have 
already  seen.  Delage  in  1887  found  that  Mysis,  Palaemon, 
and  other  forms  displayed  serious  disturbance  of  equilibrium 
when  both  eyes  and  statocysts  were  destroyed,  showing  that 
the  eyes  also  play  a  part  in  the  maintaining  of  balance  (180). 
The  eyes  have  been  found  to  cooperate  with  the  statocysts 
in  the  fiddler  crab,  Gelasimus,  and  also  in  another  decapod, 
Platyonichus  (127).  Neither  of  these  has  statoliths. 
Penceus  membraneus,  on  the  other  hand,  was  found  to  be 
permanently  disoriented  by  destruction  of  the  statocysts  or 
even  removal  of  the  statoliths,  while  blinding  produced  no 
great  disturbance,  probably  because  of  the  animal's  noc- 
turnal habits  (38,  250) .  Young  crayfish  with  the  statocysts 
destroyed  will  swim  upside  down  as  readily  as  right  side 
up  (i  u).  But  the  prettiest  evidence  for  the  static  function 
of  the  statocysts  was  obtained  when  powdered  iron  was 
substituted  for  the  mineral  bodies  in  the  open  statocysts  of 
Palaemon.  It  was  found  that  when  a  magnet  was  brought 
near,  the  animal  would  respond  by  taking  up  a  position 
corresponding  to  the  resultant  of  the  pull  of  the  magnet  and 
that  of  gravity  (407). 

Specific  righting  reactions  occur  in  many  Crustacea, 
though  in  some  cases  these  seem  to  be  merely  the  incidental 
effects  of  ordinary  locomotion.  Branchipus,  the  fairy 
shrimp,  normally  swims  upside  down ;  if  turned  right  side 
up  when  moving  along  the  bottom  of  the  vessel,  it  continues 
to  move  in  this  position  without  showing  any  disturbance 
until  it  happens  to  rise  a  little  from  the  bottom,  when  ap- 
parently the  weight  of  the  body  pulls  it  around  into  the 
usual  upside-down  position.  The  crayfish  has  two  methods 


Spatially  Determined  Reactions  189 

of  righting  itself:  a  quick  "flop"  executed  with  the  tail, 
and  a  slow  and  laborious  raising  of  itself  on  one  side  and 
tipping  over  (179). 

Many  Crustacea  show  marked  responses  to  gravity: 
for  example,  Parker  found  decided  negative  geotropism  in 
the  females  of  the  marine  copepods  whose  depth  migrations 
he  studied.  It  seems  to  be  needed  to  counteract  the  ten- 
dency of  the  animals  to  fall  to  the  bottom  by  their  own 
weight  (534).  In  certain  copepods,  light  was  observed  to 
change  the  sense  of  the  response  to  gravity,  not  by  taking 
its  place  as  a  directive  stimulus,  but  apparently  by  pro- 
ducing some  physiological  change  in  the  animals.  Their 
normal  geotropism  was  positive,  that  is,  they  had  a  ten- 
dency to  move  downwards.  In  darkness,  however,  their 
geotropism  became  negative.  They  were  also  negatively 
phototropic  to  strong  light.  If,  when  in  the  negatively 
geotropic  phase,  they  were  illuminated  from  below  by  in- 
tense light,  from  which  they  would  ordinarily  have  moved 
away,  the  change  from  negative  to  positive  geotropism 
induced  by  the  light  was  of  sufficient  influence  to  make  them 
move  downward  toward  it  (210).  Other  facts  regarding 
the  relation  of  geotropism  and  phototropism  are  mentioned 
on  pp.  209  ff . 

§  55.  Orientation  to  Gravity:  Spiders  and  Insects 

Spiders  and  insects  have  no  statolith  organs.  Bethe 
thinks  that  equilibrium  is  maintained  in  their  case  as  a 
natural  result  of  the  position  of  the  centre  of  gravity  and  the 
distribution  of  air  in  the  body.  He  supports  this  view  by 
experiments  in  which  dead  insects,  allowed  to  fall  through 
the  air,  assume  the  normal  position,  and  is  inclined  to  think 
that  all  animals  without  special  static  organs  maintain  their 
balance  in  this  way  (48).  Negative  geotropism  in  certain 


The  Animal  Mind 

insects,  as  evidenced  by  a  tendency  to  creep  from  horizontal 
planes  up  vertical  ones,  was  observed  by  Loeb  (420).  In 
light  the  eyes  of  insects  have  probably  much  to  do  with 
maintaining  equilibrium.  Certain  aquatic  insects,  in  ex- 
periments where  the  light  was  made  to  strike  them  only 
from  below,  as  soon  as  they  left  the  support  on  which  they 
were  resting,  turned  themselves  upside  down  (622). 

§  56.   Orientation  to  Gravity:   Vertebrates 

It  has  long  been  known  that  in  vertebrates  the  static 
function  resides  in  the  ear,  and  especially  in  the  semicircular 
canals  (e.g.  103,  165,  229,  263).  Various  experimenters 
have  noted  that  operations  on  the  ears  of  fishes  disturb  the 
equilibrium  of  these  animals.  Sewall,  indeed,  found  that 
section  of  the  semicircular  canals  in  the  shark  had  no  effect 
on  its  balancing  powers,  although  operations  on  the  vesti- 
bule and  ampullas  did  disturb  movement  (669) ;  and 
Steiner  got  no  effect  on  equilibrium  from  removing  the 
contents  of  the  labyrinth  (692).  Errors  in  method  and 
observation  probably  influenced  these  results.  Loeb  found 
that  severing  the  auditory  nerve  or  removing  the  statolith 
from  the  dogfish  caused  the  fish  to  incline  toward  the 
operated  side  and  to  roll  the  eyes  in  that  direction  (424). 
Total  extirpation  of  one  labyrinth  in  the  perch  was  observed 
by  Bethe  to  make  the  fish  curve  toward  the  affected  side. 
The  fish  Scardinius  showed  a  tendency  to  curve  toward  the 
opposite  side  (48) .  Lee's  experiments  on  the  dogfish  showed 
a  very  definite  relation  between  the  position  of  the  canal 
operated  upon  and  rolling  movements  of  the  fish.  Cutting 
the  front  canals  caused  the  fish  to  dive  forward,  cutting  the 
rear  canals  made  it  dive  backward,  and  cutting  the  canal 
on  either  side  made  it  roll  over  toward  that  side.  A  natural 


Spatially  Determined  Reactions  191 

explanation  of  this  behavior  is  to  suppose  that  the  absence 
of  stimulus  from  the  cut  canal  produces  the  same  effect  that 
rolling  the  fish  in  the  opposite  direction,  and  thus  diminish- 
ing the  pressure  of  the  fluid  in  the  canal,  would  produce. 
The  fish  " feels  as  if"  it  were  being  rolled  over,  and  makes 
movements  to  regain  its  equilibrium.  When  the  nerves 
supplying  the  ears  on  both  sides  were  cut,  the  fish  became 
perfectly  indifferent  to  its  position  and  would  float  upside 
down  without  any  effort  to  right  itself.  The  vestibule  and 
otoliths  of  the  fish  ear  are  thought  by  Lee  to  be  concerned 
with  static  equilibrium ;  that  is,  with  the  maintenance  of 
position  while  the  fish  is  at  rest,  while  the  canals  are  con- 
cerned with  balance  during  motion  (dynamic  equilibrium) 
(416).  It  may  be  added  that  experiments  on  the  sea  horse 
indicate  that  destruction  of  the  labyrinths  in  this  animal 
has  no  effect  on  equilibrium :  the  upright  attitude  is  due 
to  the  position  of  the  air  bladder  and  is  assumed  even  by 
dead  animals  (251). 

That  vision  may  materially  aid  in  maintaining  equilibrium 
in  vertebrates  is  indicated  by  evidence  from  various  sources, 
among  others,  the  observation  of  Bigelow  that  goldfish  in 
which  the  nerves  supplying  both  ears  had  been  cut  recovered 
after  two  or  three  weeks  and  could  swim  quite  normally 
except  when  they  were  placed  in  a  large  body  of  water  and 
made  to  swim  rapidly,  when  they  showed  no  power  of  pre- 
serving their  balance  (54).  Their  successful  performance 
of  slower  movements  was  very  likely  due  to  the  use  of 
sight. 

Sensory  impulses  from  the  body  muscles  themselves  un- 
doubtedly cooperate  with  those  from  the  semicircular  canals 
in  the  maintenance  of  balance.  They  are  evidently  in- 
volved in  the  peculiar  withdrawing  movements  by  which 
land  animals,  even  puppies,  kittens,  and  young  rats  whose 


192  The  Animal  Mind 

eyes  have  not  opened,  save  themselves  from  falling  when 
they  reach  the  edge  of  the  object  on  which  they  have  been 
crawling  (490,  683).  Water-dwelling  animals,  accustomed 
to  plunge  off  solid  supports,  lack  this  protective  instinct; 
Yerkes  showed  that  among  several  species  of  tortoises, 
some  land-dwelling,  some  amphibious,  and  some  aquatic, 
the  first  mentioned  were  much  more  reluctant  than  the 
second  to  crawl  off  the  edge  of  a  board,  and  the  second  more 
reluctant  than  the  third  (810). 


§  57.    The  Psychic  Aspect  of  Orientation  to  Gravity 

Glancing  back  over  these  examples  of  the  responses  made 
by  animals  to  gravity,  we  note  that  while  in  some  cases  the 
earth's  attraction  appears  to  act  mechanically  upon  the 
animal,  causing  the  body  passively  to  assume  a  certain 
position,  the  common  method  of  bringing  about  orientation 
seems  to  be  that  some  structure  in  the  body,  placed  in  an 
abnormal  position,  presents  a  stimulus  which  brings  about 
a  compensatory  movement.  This  structure  may  be  heavier 
particles  of  the  body  substance,  as  probably  is  the  case  in 
Paramecium ;  it  may  be  a  statolith,  or  the  fluid  in  the  laby- 
rinth ;  it  may  be  the  eyes.  In  any  case,  what  shall  we  say 
about  the  sensation  quality  involved?  Perhaps  the  re- 
actions produced  are  wholly  reflex.  Perhaps  the  statolith 
or  the  canal  fluid  produces  a  specific  sensation  quality. 
Or  perhaps,  as  Verworn  thinks,  the  sensation  quality  is 
merely  that  of  pressure  (741).  Whatever  its  nature, 
spatial  perception,  the  perception  of  the  spatial  relations 
between  several  stimuli  simultaneously  apprehended,  plays 
no  part  in  the  orientation  of  animals  to  gravity. 


Spatially  Determined  Reactions  193 

§  58.   Orientation  to  Light 

In  some  animals  light  is  sought  or  avoided  not  simply 
because  of  the  fact  that  in  certain  intensities  it  stimulates  to 
restlessness  and  activity  (photokinesis) ,  so  that  they  come  to 
rest  in  regions  illuminated  by  other  intensities ;  but  through 
a  direct  movement  of  the  animal  towards  or  away  from 
the  source  of  light.  It  is  this  type  of  response  to  which 
Loeb  and  his  followers  restrict  the  term  "  tropism."  Plants 
show  it,  both  in  the  orienting  of  their  stems  with  relation 
to  light,  and  in  the  movements  of  their  freely  swimming 
" swarm  spores."  In  the  case  of  animals,  it  is  illustrated 
by  the  behavior  of  the  sea-anemone  Actinia  cereus.  Weak 
light  causes  expansion  of  the  tentacles  of  this  organism 
perpendicularly  to  the  light  rays.  If  the  light  is  increased, 
Bohn  (86)  says  the  tentacles  "tend  to  orient  themselves  in 
the  direction  of  the  rays,  and  finally  converge  in  a  bundle 
parallel  to  that  direction,"  a  response  which  has  the  effect 
of  protecting  them  from  the  intense  light.  Again,  the  tube- 
dwelling  worm  Spirographis  spallanzanii  gradually  curves 
its  tube  until  its  mouth  end  faces  the  direction  from  which 
the  rays  of  light  come,  and  another  marine  worm,  whose 
tube  is  absolutely  stiff,  adapts  itself  to  a  change  in  the 
direction  of  the  rays  by  curving  the  newly  formed  portions 
of  the  tube  as  it  constructs  them  (42  2). 1  Sea-anemones  and 
tube-dwelling  worms  closely  resemble  plants  in  their  mode 
of  living.  In  freely  moving  animals,  where  the  oriented 
movement  is  made  in  response  to  light,  it  is  commonly 
preceded  by  body  orientation ;  that  is,  the  body  first  faces 
or  turns  tail  to  the  light,  and  the  animal  then  moves  for- 
ward. Sometimes,  however,  there  is  no  regular  body 

1  Hargitt  (288)  finds  no  such  constancy  of  orientation  in  Spirographis  as 
would  warrant  Loeb's  calling  the  motion  a  tropism. 
o 


1 94  The  Animal  Mind 

orientation ;  the  animal  moves,  for  instance,  always  away 
from  the  light,  which  means  that  it  moves  forward  if  its 
body  happens  to  be  oriented  with  the  tail  to  the  light,  or 
backward  if  its  head  happens  to  be  directed  to  the  light. 
Such  behavior  is  reported  by  Holmes  of  mosquito  larvae 
(338)  and  by  Gee  of  leeches  (257).  On  the  other  hand, 
Hadley  (274)  says  that  young  lobsters  always  orient  with  the 
head  towards  the  light,  though  they  may  move  either  away 
from  or  towards  it.  In  some  animals  with  eyes,  such  as  the 
crustacean  Daphnia,  there  is  reason  to  think  that  body 
orientation  is  primarily  an  affair  of  eye-orientation  or  fixa- 
tion. This  at  least  is  the  view  of  Radl  (621).  He  placed 
Daphnia  under  a  microscope  in  such  a  way  that  only  the 
eyes  could  be  moved.  When  the  light  coming  from  below 
was  diminished,  the  eyes  rolled  upward;  when  the  light 
coming  from  above  was  diminished,  the  eyes  rolled  down- 
ward. Holmes  (330)  observed  that  in  amphipods,  blacken- 
ing one  eye  of  a  positively  phototropic  animal  causes  a 
turning  toward  the  blackened  side,  as  if  the  animal  were 
trying  to  restore  the  missing  illumination;  similar  experi- 
ments upon  negative  animals  produced  turning  towards  the 
other  side. 

It  is  the  view  of  Loeb  (434)  that  oriented  response  of 
animals  to  light  is  wholly  analogous  to  the  same  type  of 
response  in  plants.  Since  plants  with  their  very  slow  and 
limited  movements  are  subject  more  to  light  as  a  continuous 
stimulus  than  to  sudden  changes  in  light  intensity,  orien- 
tation in  their  case  must  be  brought  about  by  the  steady  and 
continuous  action  of  the  light.  Accordingly,  Loeb  main- 
tains the  view  that  the  tropism  or  oriented  response  of 
animals  to  light  is  dependent  on  the  continuous  action  of 
the  light,  and  not  on  changes  in  light  intensity.  It  is  thus 
a  mode  of  response  that  has  nothing  in  common  with  "sen- 


Spatially  Determined  Reactions  195 

sibility  to  difference,"  which  Loeb  recognizes  as  an  indepen- 
dent form  of  reaction.  In  support  of  his  continuous  action 
theory  Loeb  lays  great  stress  on  the  proof,  by  the  botanist 
Blauuw,  that  the  "Bunsen-Roscoe  Law/'  that  is,  the  law 
that  the  effect  of  weak  light  acting  a  long  time  is  equal 
to  that  of  strong  light  acting  a  short  time,  holds  for  plants ; 
Loeb  thinks  it  holds  also  for  animals. 

The  action  of  continuous  light  in  producing  a  tropism  has 
been  explained  in  two  ways :  (i)  as  the  effect  of  the  direction 
of  the  light  rays  traversing  the  animal's  body,  and  (2)  as  the 
effect  of  having  symmetrical  points  on  the  animal's  body 
stimulated  with  unequal  degrees  of  intensity.  In  his 
earliest  discussion  of  the  subject,  Loeb  (419)  expressed 
himself  positively  in  favor  of  the  former  hypothesis.  "The 
orientation  of  animals  to  a  source  of  light  is,  like  that  of 
plants,  conditioned  by  the  direction  in  which  the  light  rays 
traverse  the  animal  tissue,  and  not  by  the  difference  in  the 
light  intensity  on  the  different  sides  of  the  animal."  Bohn, 
in  general  the  ardent  follower  of  Loeb,  urged  as  a  "funda- 
mental objection"  to  this  that  "the  'luminous  rays'  which 
strike  a  living  body  have,  save  in  wholly  exceptional  cases, 
various  directions,  being  reflected,  diffused,  and  refracted 
by  neighboring  bodies"  (80).  Moreover,  the  animal 
bodies  which  are  opaque  could  not  be  traversed  by  light 
rays.  Loeb  seems  later  to  have  abandoned  the  " direction 
theory"  of  the  tropism.  The  "intensity  theory"  was  first 
proposed  by  Verworn  (743). 

How  can  differences  in  the  intensity  of  a  stimulus  falling 
upon  symmetrical  and  opposite  points  on  an  animal's  body 
bring  about  orientation?  Let  us  call  the  two  points  a 
and  a',  a  being  a  point  on  the  right  side  of  the  animal's 
body  and  a!  a  symmetrically  placed  point  on  the  left 
side.  Suppose  the  animal  has  a  tendency  to  orient  itself 


196  The  Animal  Mind 

positively  to  the  light,  that  is,  turn  towards  the  light, 
and  suppose  a  ray  of  light  strikes  it  obliquely  from 
the  right.  Evidently  the  point  a  receives  a  greater  in- 
tensity of  the  stimulus  than  the  point  a'.  Now  if  the 
animal  is  positive  to  light,  Loeb  would  suppose  that  its 
chemical  constitution  is  such  that  light  causes,  either  by 
direct  action  on  the  muscles  or  reflexly  through  the  eyes, 
a  contraction  of  the  muscles.  Hence  the  muscles  at  point  a, 
or  controlled  through  point  a,  would  contract  more  strongly 
than  those  at  point  a' :  the  animal  in  consequence  would 
turn  towards  the  right,  that  is,  towards  the  light,  and 
would  continue  so  turning  until  the  light  struck  a  and  a' 
with  equal  intensity,  that  is,  until  it  directly  faced  the 
light.  All  subsequent  movement  would  have  to  be  directed 
straight  towards  the  light.  If  the  animal  is  negative  in  its 
response  to  light,  then  it  is  so  chemically  constituted  that 
light  causes  a  relaxation  of  the  muscles.  In  such  a  case, 
the  point  least  strongly  stimulated  would  produce  the 
strongest  muscular  contractions:  the  animal  would  turn 
towards  that  side,  and  would  continue  turning  until 
opposite  points  were  equally  stimulated,  that  is,  until  it 
headed  directly  away  from  the  light :  all  subsequent  move- 
ment would  have  to  be  in  this  direction. 

Now  Jennings  (373),  has  suggested  that  the  oriented 
reactions  of  certain  organisms,  at  least,  are  really  due  to 
changes  in  the  intensity  of  the  light,  brought  about  by  the 
animal's  own  movements.  This  view  would,  if  generalized, 
put  all  directed  light  reactions  in  the  "  sensibility  to  differ- 
ence "  class  given  to  changes  in  intensity :  the  effect  of  con- 
tinuous light  would  be  limited  to  photokinesis.  Let  us  see 
how  an  oriented  response  may  be  conceived  to  result  from 
reactions  to  changes  in  light  intensity.  In  the  Protozoa,  ac- 
cording to  Jennings  (373)  and  Mast  (463),  the  orientation  is 


Spatially  Determined  Reactions  197 

due  to  negative  reactions  given  when  the  organism  in  its  or- 
dinary swimming  movements,  which  usually  involve  turning 
from  side  to  side,  either  passes  into  a  region  of  greater  or 
less  illumination,  or  swings  its  anterior  end  "toward  or 
away  from  the  source  of  light,  so  that  it  is  shaded  at  one 
moment  and  strongly  illuminated  at  the  next."  Suppose, 
that  is,  an  animal  makes  in  its  locomotion  slight  random 
movements  of  the  head  from  side  to  side.  Suppose  that 
one  side  of  it  is  more  brightly  illuminated  than  the  other. 
If  the  animal  is  positive  to  light,  it  has  the  characteristic 
of  making  a  negative  response  whenever  its  head  end  is 
suddenly  darkened.  This  will  happen  when  the  head  end 
is  accidentally  turned  away  from  the  light;  consequently 
all  such  random  movements  will  be  checked,  while  random 
movements  of  the  head  towards  the  light  will  not  be 
checked.  Hence  the  animal  will  turn  until  its  head  points 
towards  the  light:  in  this  position  random  movements 
towards  either  side  will  be  equally  checked  because  they  will 
equally  tend  to  bring  the  head  into  a  darker  region ;  and  so 
movement  will  take  place  in  a  line  generally  towards  the 
light,  though  still  with  balanced  random  movements  to 
either  side.  If  the  animal  is  negative,  it  has  the  charac- 
teristic of  making  negative  reactions  when  the  illumina- 
tion of  the  head  is  suddenly  increased,  and  obviously  this 
will  bring  about  orientation  with  the  head  end  away  from 
the  light. 

In  Volvox  (see  page  136),  orientation  is  held  by  Oltmanns 
(528)  and  Mast  (464)  to  occur  after  this  fashion.  The 
reaction  of  a  Volvox  colony,  which  in  moderate  light  is 
positively  phototropic,  takes  place  in  consequence  of  a 
response  by  each  individual  in  the  colony  given  when,  as 
the  colony  rotates,  that  individual  passes  from  a  higher 
to  a  lower  intensity  of  light. 


198  The  Animal  Mind 

A  point  which  has  been  regarded  as  of  much  importance 
in  deciding  between  the  theories  of  Loeb  and  Jennings  on 
orientation  to  light  is  the  actual  occurrence  or  non-occur- 
rence of  random  movements.  Thus  Holmes  (334)  believes 
the  negative  orientation  of  earthworms  to  light  occurs  by 
the  checking  of  random  movements  of  the  head  towards  the 
light.  In  the  crawling  movements  stimulated  when  light  is 
thrown  upon  the  worm,  the  head  is  turned  from  side  to 
side.  If  it  happens  to  be  turned  toward  the  light,  it  is 
withdrawn.  Holmes  explains  the  observation  of  Parker 
and  Arkin  that  the  head  of  the  worm  is  much  more  apt  to 
turn  from  the  light  than  toward  it  (552),  by  saying  that 
account  was  probably  taken  here  only  of  the  first  decided 
turn  made.  He  himself  experimented  by  lowering  a  worm, 
crawling  on  a  wet  board,  while  its  body  was  in  a  straight 
line  and  contracted,  into  a  beam  of  light  at  right  angles  to 
the  body,  and  noting  the  first  movement  of  the  head.  This 
was  found  to  be  twenty-seven  times  away  from  the  light 
and  twenty-three  times  toward  the  light.  A  similar  method 
of  orientation  by  " trial  and  error"  was  observed  in  the 
leech  and  in  fly  larvae  by  Holmes  (334) . 

E.  H.  Harper,  on  the  other  hand,  working  on  the  earth- 
worm Perich&ta  bermudensis,  declares  that  if  the  light  is 
strong  enough  there  are  no  random  movements  of  the 
head  at  all,  but  the  first  movement  is  a  direct  reflex  away 
from  the  light.  When  the  light  is  only  moderate,  the 
appearance  of  random  movements  is  due  to  the  fact  that  the 
worm  is  less  sensitive  in  a  contracted  than  in  an  expanded 
state.  Locomotion  consists  in  a  series  of  contractions  and 
expansions,  and  "as  each  extension  begins  in  a  state  of 
lower  sensibility,  the  anterior  end  may  be  projected  toward 
the  light,  only  to  be  checked  when  its  increase  of  sensibility 
with  extension  makes  the  stimulus  appreciated"  (288). 


Spatially  Determined  Reactions  199 

A  similar  suggestion  that  orientation  may  occur  either  by  a 
definite  reflex  or  as  the  outcome  of  random  movements, 
according  to  the  animal's  physiological  condition,  is  to  be 
found  as  early  as  the  work  of  Pouchet  on  fly  larvae.  He 
noted  that  the  courses  taken  by  the  larvae  were  either 
straight,  "or  they  present  to  right  and  left  indentations 
due  to  the  wavering  movements  which  the  animal  makes  .  .  . 
in  a  certain  number  of  cases,  as  if  to  take  at  each  instant  a 
new  direction."  These  individual  differences  might  have 
been  accounted  for,  says  Pouchet,  by  differing  degrees  of 
hunger  in  the  larvae  (614).  Herms  (296)  reports  that  to 
low  intensities  sarcophagid  flies  orient  by  random  move- 
ments :  while  to  high  intensities  they  orient  directly. 
Bittner,  Johnson,  and  Torrey  (58)  find  that  the  earthworm 
orients  to  light  without  any  random  movements.  Hadley 
(274)  finds  the  same  true  of  larval  lobsters,  Crozier  (159) 
of  a  holothurian,  and  Bancroft  (16)  reports  of  Euglena,  a 
protozoon  which  has  the  spiral  method  of  swimming  char- 
acteristic of  so  many  animals  in  this  group,  that  "there  is 
nothing  of  trial  and  error  here :  the  organism  orients  as 
definitely  as  its  spiral  locomotion  will  allow." 

When  the  "  direction  theory  "  of  the  tropism  was  receiving 
more  attention  than  at  present,  evidence  that  an  animal 
oriented  in  response  to  the  direction  of  the  light  rather  than 
to  the  comparative  intensity  of  stimulation  on  symmetrical 
points  was  taken  as  arguing  against  Jennings  Js  view  of  the 
tropism  as  a  response  to  changes  in  light  intensity  produced 
by  random  movements.  Attempts  were  made  to  demon- 
strate the  direction  theory  experimentally.  A  typical 
experiment  of  this  type  was  that  of  Strasburger  (695), 
made  long  before  Jennings's  views  were  in  the  field,  upon 
the  swarm  spores  of  certain  plants.  He  placed  over  the 
vessel  containing  them  an  India  ink  screen,  thicker  at  one 


2OO  The  Animal  Mind 

end  so  as  to  cause  gradations  in  the  intensity  of  the  light 
reaching  the  vessel.  When  the  light  fell  perpendicularly 
through  this  screen,  the  distribution  of  the  swarm  spores 
through  the  vessel  was  nearly  uniform ;  that  is,  the  differ- 
ences of  intensity  had  no  effect.  When  the  screen  was 
removed,  and  the  light  fell  at  an  angle,  the  spores  imme- 
diately oriented  themselves  to  its  direction,  and  preserved 
this  orientation  even  when  the  screen  was  replaced.  They 
would  move  toward  the  light  even  when  by  so  doing  they 
passed  into  a  region  of  less  intense  illumination.  Jennings 
suggested  that  these  results  were  due  to  the  fact  that  "  turn- 
ing the  sensitive  anterior  end  away  from  the  source  of  the 
light"  would  diminish  the  effective  illumination  of  the 
animal  more  than  passing  into  a  slightly  less  illuminated 
region.  That  is,  the  two  ways  of  changing  the  intensity 
of  the  stimulus,  moving  forward  into  a  darker  region,  and 
turning  the  head  end  away  from  the  light,  are  here  opposed : 
the  latter  effect  is  stronger  than  the  former,  hence  the 
organisms  make  the  negative  reaction  when  the  head  end  is 
turned  from  the  light,  and  move  toward  the  shaded  region. 
"If  the  difference  in  intensity  of  light  in  different  parts 
were  increased  till  the  change  in  illumination  due  to  pro- 
gression is  greater  than  the  change  due  to  swinging  the 
anterior  end  away  from  the  source  of  light,  then  the  positive 
organisms  would  gather  in  the  more  illuminated  regions" 
(378,  page  148). 

§  59.  Influences  Affecting  the  Sense  of  Light  Orientations 

In  no  class  of  animal  responses  to  stimulation  is  the  effect 
more  dependent  upon  the  cooperation  of  a  number  of  condi- 
tions than  in  those  involving  orientation  to  light.  Many 
influences  have  been  found  to  reverse  the  sense  of  light  re- 


Spatially  Determined  Reactions  201 

actions,  transforming  negatively  phototropic  into  positively 
phototropic  animals,  and  vice  versa.  That  such  reversal 
should  occur  in  response  to  increase  or  decrease  of  the 
intensity  of  the  light  is  what  one  would  naturally  expect; 
if  a  certain  intensity  of  illumination  is  favorable  to  the  life 
processes  of  an  animal,  it  would  seem  appropriate  for  it  to 
seek  light  of  that  intensity  but  avoid  light  of  greater  in- 
tensity. Many  animals,  like  Gonionemus,  are  positive  to 
light  of  moderate  intensity  and  negative  to  strong  light 
(802).  The  females  of  the  crustacean  Labidocera  migrate 
to  the  surface  of  the  water  at  nightfall  because,  like  the 
earthworm,  they  react  positively  to  faint  light ;  and  move 
downward  at  sunrise  because  they  are  negative  in  their 
response  to  intenser  light  (534).  On  the  other  hand, 
Holmes  observed  that  Orchestia  agilis,  an  amphipod  crusta- 
cean, would,  if  brought  from  strong  to  weaker  light,  be- 
come negative  for  a  short  time;  the  meaning  of  such  a 
change  it  is  difficult  to  conjecture  (330).  Sudden  reduction 
of  light  causes  a  temporary  negative  phase  also  in  Con- 
voluta  roscojfensis  (253). 

Prolonged  action  of  light  may  alter  phototropism :  the 
" depth  migrations,"  that  is,  the  periodical  movements 
toward  and  away  from  the  surface  of  the  water,  in  the  free- 
swimming  larvae  of  the  barnacle,  Balanus,  are  due  ap- 
parently to  the  fact  that  an  exposure  of  several  hours  of 
light  will  make  positive  animals  negative,  even  though  the 
light  at  the  end  of  the  period  of  exposure  is  decidedly  fainter 
than  it  was  at  the  beginning  (269).  The  positive  reactions 
of  the  water  insect  Ranatra  increase  in  violence  the  longer 
the  light  acts;  on  the  other  hand,  after  being  kept  in 
darkness  for  several  hours,  Ranatra  is  negative  on  first 
being  taken  out  (335).  Daphnias  kept  in  darkness  for  a 
time  become  decidedly  negative  to  diffused  daylight, 


202  The  Animal  Mind 

whereas  if  kept  in  light  they  would  have  been  positive.  A 
sudden  change  in  light  intensity,  either  brightening  or 
darkening,  has  the  effect  of  making  positive  Daphnias 
temporarily  negative  (532). 

Temperature  changes  influence  response  to  light.  The  ob- 
vious suggestion  here  would  be  that  since  increased  tempera- 
ture often  accompanies  increased  intensity  of  light,  animals 
that  are  positively  phototropic  only  up  to  a  certain  degree  of 
illumination  ought  to  become  negative  when  the  tempera- 
ture is  decidedly  raised.  This,  however,  is  by  no  means 
always  the  effect  produced  by  increased  temperature. 
Strasburger's  swarm  spores  became  positive  in  higher  tem- 
peratures, negative  in  lowered  ones  (695).  Orchestia  agilis, 
which  we  have  just  seen  becomes  temporarily  negative  on 
being  brought  from  strong  into  weak  light,  may  be  made 
positive  again  if  the  water  is  slightly  warmed.  When  the 
same  animal  is  dropped  into  water,  it  becomes  strongly 
negative,  but  it  will  show  a  positive  response  if  the  water  is 
heated  almost  to  a  fatal  point  (330).  Essenberg.  (209) 
finds  that  certain  aquatic  insects  are  more  strongly  positive 
when  the  temperature  is  increased.  On  the  Other  hand, 
the  copepods  and  annelid  larvae  studied  by  Loeb  were  made 
negative  by  increased,  positive  by  lowered,  temperature. 
Other  crustaceans,  e.g.  Daphnia  (808,  185),  had  their 
responses  to  light  unaffected  by  a  fairly  wide  range  of 
temperature  changes. 

Increasing  or  decreasing  the  density  of  the  water  will  also 
affect  phototropism.  In  some  copepods  diluting  the  water 
produced  negative  responses  to  light,  while  increasing  its 
density  brought  about  those  of  the  opposite  sign  (425). 
Diluting  the  water  produced  negative  phototaxis  in  the 
larvse  of  Palaemonetes  (451).  Parker  failed  to  find  any 
such  effect  in  the  case  of  the  copepods  studied  by  him  (534). 


Spatially  Determined  Reactions  203 

W.  Ostwald  has  called  attention  to  the  possibility  that 
" internal  friction"  between  the  organism  and  the  medium 
may  affect  various  tropisms.  Freshly  caught  Daphnias 
which  are  negative  or  indifferent,  quickly  become  positive 
if  gelatine  or  quince  emulsion  is  added  to  the  water.  Since 
they  would  become  so  in  tune  anyway,  Ostwald  thinks  the 
mechanical  friction  of  the  sticky  liquid  simply  acts  as  a 
"sensibilator"  and  brings  on  this  positive  phase  sooner 

(532)- 

Change  in  the  purity  of  the  water  also  sometimes  produces 
change  of  sign  in  the  response  to  light.  The  amphipod 
Jassa,  negative  in  ordinary  sea  water,  becomes  positive  in 
foul  sea  water  (330).  The  presence  of  chemicals  is  an  in- 
fluence probably  identical  with  the  one  just  mentioned. 
Various  Crustacea  have  had  the  direction  of  their  reactions 
changed  by  carbonic  or  other  acids,  ammonium  salts,  ether, 
chloroform,  paraldehyd,  and  alcohol  (430).  Acids  and 
salts  will  reverse  the  responses  of  May  fly  larvae  (794) .  The 
ultra-violet  rays  will  make  positive  Balanus  larvae  tempora- 
rily negative  and  have  a  similar  effect  on  Daphnia  (502). 

The  state  of  hunger  or  satiety  in  an  animal  must  be  reckoned 
with :  the  caterpillars  of  Porthesia,  for  example,  are  de- 
cidedly positive  when  hungry,  much  less  so  when  fed  (423). 
The  slug  Limax  maximus,  ordinarily  negative  to  strong 
light,  is  positive  to  light  of  any  intensity  when  hungry 
(236). 

Mechanical  stimulation  is  most  striking  in  its  effect  on  light 
reactions.  Pouchet  in  1872  noted  that  fly  larvae  after 
having  been  shaken  fail  to  display  their  usual  orientation 
to  light  (614).  The  copepod  Temora  longicornis,  usually 
negative,  can  be  made  positive  by  shaking  it  (425).  Very 
curious  phenomena  of  a  similar  nature  have  been  observed 
in  the  case  of  some  Entomostraca.  Certain  individual 


204  The  Animal  Mind 

specimens  of  the  ostracod  Cypridopsis  appeared  to  be 
decidedly  positive,  others  negative.  Careful  experimental 
analysis  of  the  conditions  revealed  the  following  as  the  true 
state  of  affairs.  The  animals  are  predominantly  negative. 
But  contact  with  a  mechanical  stimulus  has  the  effect  of 
making  them  positive;  thus  a  negative  animal  that  is 
picked  up  in  a  pipette,  or  merely  comes  in  contact  with  the 
end  of  the  trough  in  swimming  away  from  the  light,  may 
become  positive.  In  course  of  time  such  a  positive  animal 
will  become  negative  of  its  own  accord,  so  to  speak,  without 
further  mechanical  stimulation,  but  such  stimulation,  if 
applied,  makes  it  negative  at  once  (718). 

Similar  experiments  upon  Daphnia  and  Cypris  gave  results 
of  the  same  general  character.  The  strong  positive  ten- 
dency of  the  former  may,  by  several  times  taking  the  animal 
up  in  a  pipette,  be  made  very  temporarily  negative;  the 
opposite  effect  could  not  be  well  tested  because  of  the  diffi- 
culty of  preserving  the  negative  state  long  enough  to  experi- 
ment on  it.  In  the  case  of  Cypris,  an  individual  temporarily 
negative  could  be  made  positive  by  picking  it  up,  but  the 
positive  phase  could  not  be  similarly  reversed.  No  other 
sudden  stimulus  produces  the  effect  which  is  thus  induced 
by  mechanical  contact  (800). 

The  effect  of  contact  was  observed  by  Holmes  in  the  ter- 
restrial amphipod  Orchestia  agilis.  The  most  permanent 
phase  of  these  animals  is  positive,  although  they  are  at  rest 
under  seaweed  on  the  beach  by  day.  But  when  they  are 
thrown  into  the  water,  they  become  strongly  negative,  no 
matter  what  the  intensity  of  the  light ;  and  to  a  considerable 
extent  this  effect  is  independent  of  the  temperature  (330, 
106).  In  the  case  of  the  copepod  Labidocera  czstiva,  being 
picked  up  in  a  pipette  will  make  the  females,  ordinarily 
positive,  negative  for  a  time.  The  males  are  normal!}/ 


Spatially  Determined  Reactions  205 

slightly  negative,  but  picking  them  up,  instead  of  reversing 
this  tendency,  increases  it  (534).  The  strong  positive 
phototropism  of  the  "water  scorpion"  Ranatra,  an  hem- 
ipterous  insect,  may  be  made  negative  by  handling,  and 
especially  by  dipping  in  water  (335). 

Periodical  changes  in  the  sense  of  response  to  light  have 
been  observed  in  animals  subjected  to  periodical  changes  in 
environment.  The  gasteropod  mollusk  Littorina  lives  on 
the  rocks  of  the  seacoast  in  regions  where  it  is  covered 
with  water  at  high  tide  and  exposed  to  the  air  at  low  tide. 
According  to  the  height  at  which  they  are  found,  some  of 
these  animals  undergo  the  alternations  of  wetness  and  dry- 
ness  at  the  ordinary  tidal  periods,  twice  a  day,  while  others 
are  reached  by  the  water  only  at  the  special  high  tides 
occurring  every  fourteen  days.  Mitsukuri  showed  that 
when  the  waves  of  a  rising  tide  cover  these  mollusks,  they 
display  negative  phototropism  and  seek  shelter  in  rock 
cavities ;  while  as  soon  as  they  are  again  exposed  to  the  air, 
their  phototropism  becomes  positive  and  they  emerge  in 
search  of  food.  Further,  he  found  that  a  Littorina  whose 
phototaxis  was  negative  could  be  made  positive  by  being 
subjected  to  the  action  of  a  stream  of  water  for  a  time  (496). 
Bohn  later  studied  the  effects  of  placing  black  or  white 
screens  near  the  animals  at  various  angles  to  their  crawling 
movements,  and  found  that  the  black  screens  exerted  an 
attractive  influence  at  certain  times,  the  white  screens  at 
others.  These  changes  in  the  "sense"  of  the  phototropism 
correspond  in  time  to  the  oscillations  of  the  tide,  even 
though  the  animals  are  studied  in  the  laboratory;  they 
tend  gradually  to  grow  less  pronounced,  however,  under 
such  circumstances.  Further,  the  level  from  which  the 
Littorinas  are  taken  influences  the  nature  of  their  response 
to  light.  Those  from  high  levels,  "which  undergo  pro- 


2o6  The  Animal  Mind 

longed  and  intense  desiccation,  habitually  move  following 
the  direction  of  the  luminous  field  in  the  negative  sense ;  the 
Littorinas  from  low  levels,  which  undergo  only  short  and 
slight  desiccation,  move,  habitually,  following  the  direction 
of  the  luminous  field  in  the  positive  sense."  The  former 
become  positively  phototropic  at  the  time  of  highest  water, 
the  latter  negatively  phototropic  at  the  time  of  low  water. 
In  all  cases,  the  tendency  is  for  the  animals  to  become  nega- 
tive at  low-water  time.  The  attraction  of  the  dark  screens 
represents  that  of  the  dark  surface  of  the  rocks  (80). 
Similar  oscillations  corresponding  to  the  periodicity  of  the 
tides  were  observed  in  the  annelid  Hedista  diver sicolor  (80), 
in  the  sea-anemone  Actinia  equina  (65),  and  in  the  hermit 
crab  (192,  194). 

It  is  probable  that  such  rhythmic  changes  in  the  sense 
of  light  response  are  due  to  the  effect  of  a  rhythmically 
recurring  cause,  such,  for  instance,  as  the  mechanical  dis- 
turbance caused  when  the  waters  of  the  rising  tide  begin 
to  agitate  the  pool  in  which  the  animal  dwells,  or  to  the 
wetness  or  dryness  of  the  tissues.  Bohn  has  suggested  this 
explanation  for  the  oscillation  of  Hedista,  just  mentioned. 
He  supposes  that  when  the  annelid  is  dry,  light  has  the 
power  of  exciting  muscular  movements,  that  is,  a  kinetic 
effect.  This  means  that  when  the  worms  have  accidentally 
crept  into  the  shade  they  come  to  rest.  If  one  eye  has  its 
illumination  diminished,  there  is  an  inhibition  of  muscular 
activity  on  that  side,  and  consequently  a  turning  in  that 
direction.  At  the  period  of  high  tide,  when  the  muscles 
are  wet,  the  action  of  light  on  the  animal  is  inhibitory  and 
the  above  phenomena  are  reversed  (80).  Heat  and  dry- 
ness  make  terrestrial  amphipod  crustaceans  positive  to 
light;  cold  and  wetness  make  them  negative  (106). 

The  state  of  rest  or  movement  is  still  another  factor.     The 


Spatially  Determined  Reactions  207 

"mourning  cloak"  butterfly,  Vanessa  antiopa,  on  coming  to 
rest  in  bright  sunlight,  orients  itself  with  the  head  away 
from  the  light.  When  it  moves,  on  the  other  hand,  it 
flies  toward  light  of  any  intensity  (537).  Bohn  also  has 
noted  that  certain  butterflies  orient  themselves  when 
alighted  in  such  a  way  that  the  posterior  part  of  the  eyes 
is  toward  the  light.  When  in  this  position  there  is  a  ten- 
dency for  the  wings  to  be  spread  apart,  while  when  the  insect 
is  facing  the  light  the  wings  are  closely  folded  (82).  The 
effect  on  the  wings  was  noted  in  Vanessa  also,  and,  it  is 
suggested,  may  have  some  function  in  bringing  the  sexes 
together  (537) .  The  pomace  fly  when  at  rest  is  not  oriented 
at  all.  Light  exerts  upon  it  merely  the  effect  of  stimulating 
it  to  movement,  a  kinetic,  not  a  directive,  effect.  When 
the  movement  has  been  started,  however,  it  is  directed 
toward  the  light.  But  owing  to  the  kinetic  influence  of 
the  light,  when  the  insects  have  been  long  exposed  to  sun- 
light they  tend  to  come  to  rest  in  the  more  shaded  portions, 
with  their  heads  away  from  the  light,  for  this  is  the  posi- 
tion in  which  they  are  least  stimulated  to  movement. 
The  kinetic  effect  increases  with  the  intensity  of  the  light, 
but  its  directive  effect,  through  which  orientation  is 
secured  after  the  movement  is  started,  was  at  least  in  one 
case  lost  under  intense  light  (116).  Brundin  (106)  has 
suggested  that  the  effect  of  mechanical  stimulation  in 
reversing  light  reaction  may  be  due  to  the  state  of 
activity  it  induces. 

The  background,  finally,  sometimes  determines  the  sense 
of  the  reaction.  Keeble  and  Gamble  found  that  while  the 
crustacean  Hippolyte  varians  would  move  toward  the  light 
whether  it  was  on  a  white  or  black  background,  Macromysis 
inermis  was  negative  on  a  white  ground  and  positive  on  a 
black  ground  (396). 


208  The  Animal  Mind 

§  60.   The  Psychic  Aspect  of  Orientation  to  Light 

The  behavior  of  an  organism  which,  by  the  unequal 
contraction  of  symmetrically  placed  muscles,  is  forced 
around  into  a  position  directly  facing  or  turning  tail  to 
light,  the  light  acting  as  a  continuous  stimulus  and  not 
through  changes  in  intensity,  is  without  any  parallel  in 
human  experience,  and  hence  suggests  no  psychic  accom- 
paniment. Yet  there  seems  to  be  a  considerable  amount 
of  evidence  that  such  a  type  of  reaction  does  occur,  given 
the  proper  amount  of  stimulus  and  the  proper  physiological 
condition  in  the  animal.  It  is  a  fact  of  much  interest, 
however,  that  when  we  reach  organisms  beyond  a  cer- 
tain point  in  the  ascending  scale  of  complexity,  the  tropic 
type  of  response  to  light  begins  to  give  place  to  more  variable 
responses  suggesting  analogies  with  our  own  behavior. 
The  individual  experience  of  an  animal  strongly  modifies 
its  tropisms,  as  we  shall  see  in  a  later  chapter.  Brundin 
(106)  says  that  in  certain  amphipod  crustaceans  which  he 
studied,  the  "mode  of  behavior  exhibits  a  transition  from 
the  stage  at  which  the  creature  is  at  the  mercy  of  its  en- 
vironment to  a  stage  at  which  it  is  beginning  to  hold  its 
own  against  the  forces  which  have  shaped  it."  Quite 
possibly,  however,  the  ability  to  modify  tropic  response 
by  individual  experience  is  found  in  all  animals,  and  not 
merely  in  those  above  a  certain  stage ;  it  does  seem  to 
be  true,  though,  that  the  tropisms  are  more  readily  over- 
thrown by  other  influences,  the  higher  the  animal.  Thus 
Holmes  (337)  says  of  fiddler  crabs  that  phototropism  is 
easily  overcome  by  fear ;  although  they  are  strongly  posi- 
tive they  will  run  away  from  a  moving  light.  "Light," 
he  says,  "is  followed  much  as  an  animal  pursues  any  other 
object  of  interest " ;  and  Turner  (728)  has  made  similar  com- 


Spatially  Determined  Reactions  209 

ments  on  the  behavior  of  certain  insects  to  light.  Bohn 
says  of  the  mollusk  Littorina  that  when  its  tissues  are 
neither  very  wet  nor  very  dry,  it  ceases  to  respond  with  a 
fatal  necessity  to  light;  "the  animal  seems,  as  it  were,  to 
disengage  itself  from  the  influence  of  external  forces,  seems 
no  longer  to  behave  like  a  pure  machine :  it  goes  to  the 
stones  and  seaweed  where  it  may  find  shelter  and  nourish- 
ment as  if  it  saw  and  was  conscious  of  them"  (80). 

§  61.   Mutual  Influence  of  Light  and  Gravity  Orientations 

Orientation  to  light  and  orientation  to  gravity  are  not 
without  mutual  influence  in  determining  the  behavior  of 
an  animal.  Supposed  instances  of  this  have  been  noted 
in  the  case  of  the  periodically  changing  geotropism  of 
Convoluta  roscojfensis  (253)  and  in  the  copepods  observed 
by  Esterly  (210).  The  relations  of  gravity  and  light  re- 
sponses in  the  larvae  of  the  squid,  a  cephalopod  mollusk, 
seem  to  be  as  follows.  The  larvae  have  a  tendency  to  rise 
to  the  surface  of  the  water  both  in  darkness  and  in  light, 
suggesting  negative  geotropism.  Two  test  tubes  were 
arranged  by  Loeb,  one  lying  horizontally  and  at  right  angles 
to  a  window,  the  other  inclined  at  an  angle  of  45  degrees 
from  the  upright  position,  and  with  the  upper  end  directed 
away  from  the  window.  Larvae  were  placed  in  both  tubes ; 
those  in  the  former  showed  positive  phototropism  by  collect- 
ing at  the  end  nearest  the  window,  but  those  in  the  latter 
gave  evidence  that  their  negative  geotropism  was  stronger 
than  their  positive  phototropism  by  rising  to  the  upper  end, 
although  it  was  farthest  from  the  source  of  light  (428). 
It  is  not  usual  for  geotropism  thus  to  come  off  victorious  in 
a  contest  with  other  tendencies.  Jennings  says,  "As  a 
general  rule  the  reaction  to  gravity  is  easily  masked  by 


2io  The  Animal  Mind 

reactions  to  other  stimuli"  (378,  p.  150).  In  the  mol- 
lusks  observed  by  Bonn,  the  tendency  in  ascending  or 
descending  the  rocks  is  to  orient  the  body  in  the  line  of  the 
greatest  slope.  When  light  and  gravity  are  acting  together 
upon  the  animal,  its  movement  seems  to  be  a  resultant  of 
the  two,  but  if  the  mollusk  is  made  to  move  on  a  vertical 
plane,  gravity  thus  exerting  its  maximal  force,  the  influ- 
ence of  the  light  disappears  altogether ;  and  if  the  animal 
is  put  in  an  upside-down  position  by  further  tipping  of  the 
surface,  the  sense  of  its  phototropism  is  reversed ;  that  is, 
it  may  be  repelled  instead  of  attracted  by  a  dark  screen  (80). 
The  fairy  shrimp,  Branchipus,  is  positively  geotropic  in 
light,  negatively  geotropic  in  darkness  (454). 

A  curious  tendency  has  been  noted  by  many  observers  in 
insects  with  both  eyes  blinded ;  namely,  to  fly  straight  up 
into  the  air.  Forel  thought  they  did  so  because  in  no  other 
direction  could  they  escape  obstacles  (231) ;  but  this  fact 
they  would  have  to  learn  by  experience,  for  which,  in  some 
cases  at  least,  they  do  not  take  time.  Plateau  believed  the 
rising  into  the  air  was  due  to  sensations  produced  by 
the  action  of  the  light  on  the  surface  of  the  body,  leading  the 
insects  in  the  direction  of  the  strongest  light,  which  usually 
comes  from  above.  He  supported  this  view  by  showing 
experimentally  that  a  blinded  insect  would  not  rise  if  set 
free  at  night,  while  on  the  other  hand,  if  liberated  in  a 
lighted  room,  it  would,  in  spite  of  the  blinding,  fly  toward 
the  light  or  the  lightest  part  of  the  ceiling  (596,  599). 
In  the  butterfly  Vanessa,  Parker  thinks  the  rising  due 
to  negative  geotropism,  as  the  insect  flew  upward  in 
a  darkened  room  (537).  Axenfeld  suggested  that  it 
might  be  caused  by  light  penetrating  the  integument 
of  the  head  (9). 


Spatially  Determined  Reactions  211 

§  62.   Orientation  to  Other  Forces 

One  force,  which,  as  was  noted  in  Chapter  III,  produces 
orientation,  namely,  the  electric  current,  we  shall  leave  out 
of  account.  It  is  not  a  stimulus  to  which  animals  are  nor- 
mally subject,  and  though  its  action  on  living  matter  is  of 
great  interest  to  the  physiologist,  the  comparative  psycholo- 
gist's difficulty  in  rinding  a  psychic  interpretation  for  the 
facts  may  justify  setting  them  aside.  Similar  considera- 
tions apply  to  orientation  to  centrifugal  force.  There 
remain  the  orientations  that  have  been  termed  respectively 
"rheotropism"  and  "anemotropism,"  responses  to  cur- 
rents of  water  and  to  currents  of  air. 

The  tendency  shown  by  many  aquatic  animals  to  orient 
themselves  with  head  up-stream,  and  to  swim  against  the 
current,  was  formerly  thought  to  be  a  response  to  the  pres- 
sure exerted  by  the  current  —  a  reaction  leading  the  ani- 
mal to  resist  pressure.  Lyon,  however,  pointed  out  that 
this  explanation  assumes  rheotropism  on  the  animal's 
part.  It  is  because  the  animal  opposes  the  current  that 
the  current  exerts  any  pressure.  If  it  merely  allowed 
itself  to  be  carried  passively  along,  and  if  the  current  sur- 
rounding the  animal  flowed  with  uniform  velocity  in  all 
its  parts,  no  stimulus  whatever  could  be  exerted  by  the 
water  pressure  (448).  It  seems  probable  that  eyeless 
animals  do  not,  as  a  matter  of  fact,  orient  themselves  against 
a  current  of  this  sort,  and  that  rheotropism  in  their  case 
occurs  when  a  current  of  unequal  velocity  disarranges 
their  movements,  or  when  they  are  in  contact  with  a  solid 
body.  Thus  Jennings  has  suggested  that  in  Parame- 
cium  the  reaction  is  due  to  the  fact  that  unless  the  animal 
has  its  head  to  the  current,  the  flow  of  the  latter  will  inter- 
fere with  the  normal  backward  stroke  of  the  cilia,  causing 


212  The  Animal  Mind 

negative  reactions,  until  the  disturbance  is  removed  by 
proper  orientation  (378,  p.  74).  In  animals  with  eyes, 
however,  there  is  reason  to  think  that  apparent  rheotropism 
is  largely  an  affair  of  vision.  Lyon's  theory  of  rheotropism 
in  fishes  is  that  the  fish  orients  itself  and  swims  in  such 
a  way  that  its  surroundings,  the  bottom  of  the  stream, 
for  example,  shall  appear  to  the  sense  of  sight  to  be  at  rest, 
an  hypothesis  which,  as  we  shall  see,  was  adopted  by  Radl 
to  explain  the  "hovering"  of  insects  in  one  place  (622). 
Lyon  supports  it  by  experiments  where  the  bottom  or 
sides  of  the  aquarium  were  caused  to  move  in  the  absence 
of  any  current  in  the  water,  and  the  fish  was  found  to  follow 
them.  When  the  fish  was  placed  in  a  revolving  glass 
cylinder,  it  followed  the  revolutions,  although  there  was 
a  slow  current,  of  course,  in  the  same  direction,  against 
which,  on  the  pressure  theory,  the  fish  should  have  moved. 
Still  more  decisive  was  the  experiment  where  young  fish 
were  placed  in  a  corked  bottle  full  of  water  which  was  sub- 
merged and  put  near  a  wall  covered  with  algae.  When  the 
bottle  was  moved  in  one  direction,  all  the  fish  went  to  the 
opposite  end,  although  no  current  could  have  been  pro- 
duced. Again,  a  wooden  box  with  ends  of  wire  netting, 
the  bottom  covered  with  gravel  and  the  sides  with  sea- 
weed, was  used ;  fish  (Fundulus)  were  placed  in  it,  and  the 
box  was  held  lengthwise  in  a  strong  current.  The  fish 
oriented  themselves,  but  as  soon  as  the  box  was  released 
and  allowed  to  float  away,  they  lost  their  orientation, 
though  their  relation  to  the  current  was  in  no  way  altered. 
Blind  fish,  Lyon  found,  oriented  themselves  by  touch, 
sinking  to  the  bottom.  There  does,  however,  appear  to 
be,  in  some  cases,  a  genuine  pressure  reaction  to  current, 
for  when  water  is  rushing  through  a  small  hole  into  a  tank 
containing  blind  fish,  they  keep  their  heads  to  the  current 


Spatially  Determined  Reactions  213 

without  touching  anything.  Here  the  different  parts  of 
the  stream  have  different  velocity,  and  pressure  stimuli 
are  actually  applied  to  the  skin.  There  must  be  pressure 
reaction,  also,  when  fish  actually  swim  up-stream  instead 
of  merely  maintaining  their  places  against  a  current  (272). 
Such  a  reaction  was  displayed,  probably,  by  some  shrimps 
which,  being  in  the  water  with  the  fish  in  the  revolving 
tank  experiment,  did  swim  against  the  current  instead  of 
with  it  (448). 

Some  very  interesting  behavior  touching  on  this  same 
point  was  observed  by  Garrey  in  a  school  of  the  little  fish 
called  sticklebacks.  He  noted  that  if  any  object  was 
moved  along  the  side  of  the  aquarium  containing  them, 
the  whole  school  would  move  along  a  parallel  line  in  the 
opposite  direction.  If  an  individual  fish  happened  to  be 
heading  directly  toward  the  object,  it  would  turn  in  the 
opposite  direction  from  the  one  in  which  the  object  was 
moved ;  if  it  was  heading  somewhat  in  the  opposite  direc- 
tion already,  it  would  turn  farther  in  that  direction  until 
parallel  with  the  object's  line  of  motion ;  if  it  was  heading 
somewhat  in  the  same  direction  as  the  object,  it  would 
"back  off  hesitatingly,"  and  reverse  itself  by  a  turn  in 
either  direction,  usually  taking  the  way  around  toward 
which  it  was  already  partially  headed,  if  the  object  was 
rapidly  moved,  but  the  other  way  around  if  the  object's 
motion  was  slow.  At  first  sight  this  behavior  seems  to 
display  an  instinct  precisely  opposite  to  that  of  keepmg 
the  visual  field  constant.  Yet  the  sticklebacks,  when 
placed  in  a  cylindrical  glass  tank  inside  of  a  black  and  white 
striped  vessel,  moved  with  the  latter  when  it  moved, 
proving  that  they  possessed  the  usual  tendency  shown  by 
Lyon  to  be  involved  in  rheotropism.  Garrey  points  out 
that  movement  in  the  opposite  direction  is  produced  not 


214  The  Animal  Mind 

when  the  whole  visual  field  moves,  but  when  it  is  at  rest, 
and  one  object  in  it  moves.  Can  it  be,  he  asks,  that 
the  moving  object  " fixes  the  attention"  of  the  fish  and 
produces  an  apparent  motion  of  the  background  in  the 
opposite  direction,  which  motion  the  fish  follows?  (254.) 

Rheotropism  in  water  arthropods  may  be  similarly  ac- 
counted for,  and  in  the  opinion  of  Radl,  this  same  tendency 
explains  the  habit  swarms  of  insects  have  of  hovering  over 
the  same  place,  a  phenomenon  which  Wheeler  thought 
might  be  due  to  odors  emanating  from  the  soil  (780).  In- 
sects will  often  be  found  to  follow  an  object  over  or  under 
which  they  are  grouped  in  the  air,  if  it  be  moved  (622). 
Swarms  of  insects  may  be  noted  in  the  air  over  a  country 
road,  following  its  windings  and  apparently  oriented  by  the 
contrast  between  the  road  and  the  dark  banks  on  either 
side.  When,  however,  resting  insects  turn  so  as  to  keep 
their  heads  to  the  wind,  the  reaction  is  evidently  really 
due  to  the  wind  and  not  to  their  visual  surroundings  (646). 
Probably  the  disturbance  to  their  wings  produced  by  any 
other  position  causes  them  to  rest  only  in  the  " head-on" 
orientation. 

The  responses  of  animals  to  different  intensities  of  heat 
seem  not  to  involve  a  definite  orientation  of  the  body.  A 
temperature  above  the  optimum  produces  wandering  move- 
ments, which  cease  when  the  animal  happens  to  reach  the 
proper  temperature  (480,  483,  808). 


CHAPTER  IX 

SPATIALLY  DETERMINED  REACTIONS  AND  SPACE  PERCEP- 
TION (continued) 

§  63.   Class  III :  Reactions  to  a  Moving  Stimulus 

SPECIALIZED  response  to  a  stimulus  in  motion,  that  is, 
one  which  successively  affects  several  neighboring  points  on 
a  sensitive  surface,  is  also  frequently  met  with  in  animal 
behavior.  Its  usefulness  is  obvious :  a  stimulus  in  motion 
is  very  commonly  a  living  creature,  hence  either  an  enemy 
or  food.  In  any  case  it  must  be  reacted  to  with  extreme 
promptness.  Reactions  of  this  class  may  be  distinguished 
as  tactile  or  visual  according  as  the  moving  stimulus  is 
mechanical  or  photic. 

We  find  good  examples  of  specialized  reactions  to  motile 
touch  in  the  ccelenterates.  The  sea-anemone  Aiptasia  gives 
its  most  violent  reaction,  involving  all  the  tentacles  at  once, 
when  touched  by  a  moving  object  (521).  The  medusa 
Gonionemus  makes,  in  the  case  of  a  moving  mechanical 
stimulus,  its  single  exception  to  the  rule  of  responding  by 
the  feeding  reaction  to  edible  substances  only.  The  tenta- 
cles are  wound  corkscrew  fashion  about  a  glass  rod 
drawn  across  them,  they  bend  in  toward  the  mouth,  and 
the  bell  margin  bearing  them  contracts ;  the  feeding  reac- 
tion goes  no  further,  however.  But  the  response  is  dif- 
ferentiated from  that  to  any  other  form  of  stimulation  by 
its  greater  speed :  the  reaction  time  is  from  .3  to  .35  of  a 
second,  compared  with  .4  to  .5  of  a  second  for  other  stimuli 

215 


216  The  Animal  Mind 

(802).  Special  vigor  and  speed  generally  characterize 
reactions  to  contact  with  moving  objects.  In  eliciting  the 
scratch-reflex  of  dogs,  an  object  drawn  along  the  skin  is 
decidedly  more  effective  than  one  pressed  against  the  skin 
for  the  same  length  of  time  (681,  p.  184).  The  physio- 
logical effect  is  probably,  Sherrington  says,  the  same  as 
that  involved  in  the  "summation"  of  successive  slight 
stimuli  applied  at  the  same  point.  As  is  well  known,  the 
latter  will  bring  about  a  response  of  considerable  violence, 
though  each  stimulus  acting  alone  would  apparently  be 
without  effect. 

Is  it  likely  that  these  responses  to  moving  stimuli  in 
contact  with  the  skin  involve  the  perception  of  movement 
as  a  form  of  space  perception ;  that  is,  a  perception  of  the 
successive  positions  occupied  by  the  stimulus  and  their 
relative  direction?  I  think  we  may  say  that  they  prob- 
ably do  not,  in  the  lower  animal  forms  at  least.  And  a 
chief  reason  for  saying  so  lies  in  the  fact  that  the  reactions 
are  so  rapid.  To  perceive  the  spatial  relations  of  stimuli, 
or  any  other  relations,  is  a  process  not  favored  by  great 
speed  of  response.  The  quicker  the  reaction,  the  less  clear 
the  perception  of  its  cause :  such  seems  to  be  the  general 
law.  The  sensation  accompanying  contact  with  a  moving 
object  may  differ  in  intensity  from  that  accompanying 
a  resting  stimulus ;  it  may,  in  the  lower  forms,  differ  quali- 
tatively in  some  way  not  represented  in  our  own  experi- 
ence, but  it  can  hardly  be  connected  with  the  more  complex 
psychic  processes  involved  in  any  form  of  space  perception. 

In  vision,  also,  there  are  special  arrangements  for  react- 
ing to  moving  stimulation.  The  sensitiveness  of  many 
animals  to  changes  of  light  intensity,  although  not  a  direct 
adaptation  to  the  spatial  characteristics  of  a  stimulus,  serves 
the  same  purpose,  for  changes  in  light  intensity  are  oftenest 


Spatially  Determined  Reactions  217 

brought  about  by  objects  in  motion.  In  the  mollusk  Pecten 
varius,  a  transition  from  shadow  vision  to  movement  vision 
is  illustrated :  the  animal  closes  its  shell  when  a  shadow 
is  moved  so  as  to  fall  on  its  eye  spots  in  rapid  succession 
(628).  Generally  speaking,  the  simple  invertebrate  eye, 
however,  is  adapted  to  respond  to  changes  in  light  intensity 
rather  than  to  moving  objects.  Plateau  found  that  cater- 
pillars, which  have  only  simple  eyes,  could  see  moving  ob- 
jects no  better  than  those  at  rest  (597),  and  Willem  was 
inclined  to  think  snails  saw  resting  objects  better  than 
moving  ones  (788).  On  the  other  hand,  the  compound 
eye  (see  page  219)  is  specially  formed  to  be  affected  by  mov- 
ing stimuli.  The  crayfish  will  react  to  anything  of  fairly 
good  size  in  motion,  but  is  apparently  unable  to  avoid  sta- 
tionary objects  in  its  path  (40) .  The  poor  vision  of  the  com- 
pound eye  for  resting  objects  is  shown  by  the  ease  with  which 
insects  may  be  captured  if  the  movements  of  the  captor 
are  very  slow.  They  may  be  readily  approached,  also, 
if  the  movements  are  all  in  the  line  of  sight,  that  is,  directly 
toward  the  insect,  so  that  successive  facets  of  the  compound 
eye  are  not  affected,  as  would  be  the  case  in  lateral  move- 
ments. Let  the  reader  try  bringing  the  hand  slowly  straight 
down  over  a  fly,  and  see  how  much  closer  he  can  come  before 
the  fly  is  disturbed  than  he  can  if  the  hand  is  moved  from 
side  to  side.  Plateau,  from  experiments  on  different  orders 
of  insects,  concludes  that  "visual  perception  of  movement" 
is  best  developed  in  the  Lepidoptera  (moths  and  butter- 
flies), Hymenoptera  (ants,  bees,  and  wasps),  Diptera  (flies), 
and  Odonata  (dragon-flies) ;  that  the  distance  at  which 
movements  can  be  seen  does  not  exceed  two  metres,  and 
averages  1.5  metres  for  diurnal  Lepidoptera,  58  cm.  for 
Hymenoptera,  and  68  cm.  for  Diptera  (599). 

It  is  possible  that  response  to  a  moving  stimulus  received 


218  The  Animal  Mind 

through  the  eye  may  be  accompanied  by  spatial  perception 
of  movement,  although  if  the  eye  is  compound,  the  experi- 
ence must  differ  from  our  own  visual  movement  perception. 

§  64  Class  IV:  Reaction  to  an  Image 

By  an  image  is  meant  the  perception  of  simultaneously 
occurring  but  differently  located  stimuli  as  having  certain 
spatial  relations  to  each  other.  Through  its  means,  or 
that  of  the  nervous  processes  underlying  it,  there  arises 
the  possibility  of  adapting  reaction  not  merely  to  the  loca- 
tion of  a  single  stimulus,  but  to  the  relative  location  of 
several  stimuli.  Responses  may  thus  be  adjusted  not 
only  to  the  direction  of  an  object  but  to  its  form.  On  the 
basis  of  such  adjustments  a  whole  new  field  of  possible 
discriminations  is  opened  up. 

The  commonest  arrangement  for  the  production  of  a 
visual  image  is  the  double  convex  lens,  which  collects  the 
rays  of  light  diverging  in  their  reflection  from  an  object 
and  brings  them  together  again  upon  the  sensitive  retina. 
The  lenses  found  in  many  simple  invertebrate  eyes  seem, 
however,  very  ill  adapted  to  the  image-producing  function. 
It  is  probable  that  they  serve  rather  to  intensify  the  effect 
of  the  light  rays  by  bringing  them  together,  than  to  give 
a  clear-cut  image  (523).  In  the  eye  of  certain  inverte- 
brates, such  as  the  Nautilus,  a  cephalopod  mollusk,  while 
there  is  no  lens,  the  opening  admitting  the  light  rays  is 
so  small  that  an  inverted  image  might  be  formed  through 
it,  such  as  may  be  obtained  through  a  pinhole.  It  is  un- 
likely, however,  that  this  eye  is  really  an  image-producing 
organ.  Hesse  includes  under  image-forming  eyes  only 
the  camera  or  convex-lens  eye,  the  mosaic  eye,  and  the 
superposition  eye.  The  last  is  a  peculiar  form  of  com- 


Spatially  Determined  Reactions 


219 


pound  eye  where  light  can  pass  from  one  section  to  another, 
and  where  the  image  is  formed  by  the  cooperation  of  vari- 
ous refracting  bodies  (324). 

The  simplest  and  vaguest  conceivable  visual  image  would 
be  that  of  a  visual  field  whose  different  parts  should  differ 
in  brightness. 
An  eye  capa- 
ble of  furnish- 
ing indications 
merely  of  the 
direction  from 
which  the  great- 
est illumination 
comes  might 
produce  this 
kind  of  an  im- 
age, which 
would  of  course 
not  allow  the 
perception  of 
objects,  only 
that  of  bright- 
ness distribution.  The  compound  eye  found  in  crus- 
taceans and  insects  would  seem  to  be  adapted  chiefly 
for  the  perception  of  light  direction  and  of  moving  stimuli. 
It  consists  essentially  of  a  number  of  simple  eyes  so  crowded 
together  as  to  produce  a  common  faceted  cornea,  each 
facet  belonging  to  an  eye.  These  facets  are  lens  shaped, 
and  back  of  each  lies  a  refractile  crystalline  cone.  Behind 
these,  in  turn,  are  nervous  structures,  the  rods  or  retinulas, 
each  separated  from  its  neighbors  by  a  pigment  sheath. 
Light  rays  passing  through  each  corneal  facet  probably 
produce  a  single  spot  of  light  on  the  retinula,  and  the 


FIG.  ii.  — Diagrammatic  representation  of  the  compound 
eye  of  a  dragon-fly.  C,  cornea;  K,  crystalline  cone; 
P,  pigment;  R,  nerve  rods  of  retina;  Fb,  layer  of 
fibres;  G,  layer  of  ganglion  cells;  Rf,  retinal  fibres; 
Fk,  crossing  of  fibres.  After  Claus. 


22O  The  Animal  Mind 

total  image  may  thus  be  a  mosaic  formed  of  these  spots 
(Fig.  n). 

We  have  already  seen  that  the  orientations  of  certain 
animals  to  light  seem  to  be  produced  through  a  tendency 
to  take  such  a  position  that  the  two  eyes  shall  be  equally 
illuminated.  If  the  two  visual  fields  are  combined  in  the 
case  of  such  animals,  as  they  are  in  our  own  binocular 
vision,  under  ordinary  conditions  the  oriented  position 
would  give  a  field  whose  brightness  is  uniform  throughout, 
while  any  other  position  would  give  greater  brightness 
at  one  side  of  the  field.  If  they  are  not  combined,  if 
there  is  no  binocular  vision,  we  cannot  imagine  what  the 
resulting  perception  is.  In  the  case  of  the  starfish,  we 
have  an  animal  which  seems  to  "see"  a  vertical  white  wall 
or  dark  wall  that  does  not  cast  any  actual  shadow  upon  it ; 
the  starfish  will  direct  its  movements  to  or  from  such  ob- 
jects. Since  the  starfish  has  only  eye-spots  on  the  tips  of 
its  arms,  with  no  arrangements  for  the  formation  of  an 
image,  and  since  the  eye-spots  are  not  arranged  close  enough 
together  so  that  differences  of  illumination  in  different  parts 
of  a  field  could  be  represented  by  the  different  illumination 
of  different  eye-spots,  we  can  explain  the  reaction  to  walls 
only,  as  Cowles  (156)  does,  by  supposing  that  those  eye- 
spots  and  portions  of  the  body  nearest,  say,  a  white  wall, 
are  more  strongly  illuminated  than  those  furthest  away. 
The  response  would  then  be  one  to  different  intensities  of 
stimulation  on  different  parts  of  the  body,  and  these  dif- 
ferences would  not  be  seen  as  composing  a  visual  field. 

That  the  direction  from  which  the  light  comes  influences 
ants  in  finding  their  way  is  the  opinion  of  Lubbock  (441), 
Turner  (722  a),  and  Santschi  (654).  The  first  named  found 
that  ants  which  had  learned  the  way  back  to  an  artificial 
nest  were  confused  when  two  candles  which  had  stood 


Spatially  Determined  Reactions  221 

near  the  nest  were  moved  to  the  opposite  side.  Turner 
made  a  similar  observation,  and  Santschi  suggests  that 
the  compound  eye  may  perceive  the  direction  of  light  by 
acting  as  a  kind  of  sundial.  He  was  able  to  make  ants  re- 
verse their  course  when  he  altered  the  light  direction  by 
the  use  of  mirrors. 


§  65.   Methods  of  Investigating  the  Visual  Image 

Various  methods  of  solving  the  problem  as  to  the  nature 
and  accuracy  of  an  animal's  visual  images  have  been  used. 
One  method  consists  in  a  study  of  the  sense-organ  itself, 
removed  from  the  body.  For  example,  Petrunkevitch 
(575)  has  thus  investigated  the  sense  of  sight  in  spiders. 
These  animals  do  not  have  the  compound  eye,  but  a  num- 
ber of  simple  eyes  placed  in  groups.  By  a  careful  measure- 
ment of  the  possible  minimal  angles  of  vision  in  two  spiders, 
Phidippus  and  Lycosa,  the  conclusion  is  reached  that 
while  a  creeping  insect  about  one  square  centimeter  in 
size  would  be  to  the  human  eye  so  clearly  visible  at  a  dis- 
tance of  three  metres  that  its  species  could  be  recognized, 
it  would  be  only  an  indefinite  moving  speck  to  the  eye 
of  Phidippus  and  wholly  beyond  the  range  of  vision  of 
Lycosa. 

Again,  inferences  are  drawn  as  to  the  visual  powers  of 
animals  from  miscellaneous  peculiarities  of  behavior. 
Thus  Petrunkevitch  (576)  reports  that  a  male  spider  of 
the  species Dysdera  crocata,m  the  courting  stage,  "watched" 
the  movements  of  the  end  of  a  hatpin  with  which  the  ob- 
server was  breaking  clumps  of  earth,  and  when  the  move- 
ment ceased  the  spider  approached  the  spot  and  scratched 
it  with  his  front  legs.  The  sight  of  a  female  spider  digging 
had  the  same  effect  upon  him,  so  evidently  the  visual 


222  The  Animal  Mind 

image  which  he  received  was  hardly  more  definite  than 
one  of  general  size  and  movement.  Bauer  (28)  reports 
of  the  mollusk  Pecten,  which  has  eyes  of  peculiar  and  com- 
plicated structure,  that  when  a  small  quickly  moving  shadow 
is  cast  upon  it,  the  tentacles  are  quickly  withdrawn; 
large  or  slowly  moving  shadows  have  no  effect,  but  a  small, 
slowly  moving  shadow  makes  the  animal  stretch  its  ten- 
tacles and  eyes  towards  the  shadow.  In  this  way,  Bauer 
thinks,  it  is  enabled  to  ascertain  the  nearness  of  its  worst 
enemy,  a  starfish :  apparently  he  supposes  that  the  move- 
ment of  the  eyes  towards  the  shadow  gives  an  opportunity 
for  visual  perception  of  its  form  or  characteristic  move- 
ments. Wenrich  (777)  has  recently  obtained  the  following 
evidence  of  the  formation  of  an  image  in  Pecten.  The 
bivalve  normally  responds  only  to  a  decrease  in  illumina- 
tion, not  to  an  increase.  If  a  white  card  is  moved  across 
a  black  one,  the  card  being  not  less  than  fifteen  millimeters 
square  and  its  distance  not  greater  than  thirty-five  centi- 
metres, Pecten  responds  by  closing  its  shell,  although  the 
illumination  is  increased  rather  than  diminished. 

The  chief  lines  of  evidence,  however,  from  which  the 
nature  of  the  visual  image  can  be  concluded  are  three :  ex- 
periments on  the  visual  perception  of  size,  experiments 
on  the  visual  perception  of  form,  and  experiments  or  obser- 
vations on  the  recognition  of  visual  landmarks  in  homing. 

§  66.   The  Visual  Perception  of  Size 

Bonn's  observations  on  the  mollusk  Littorina  show  that 
its  reactions  are  influenced  by  the  size  of  the  illuminated 
or  darkened  surface,  as  well  as  by  the  intensity  of  the  light. 
When  neither  very  wet  nor  very  dry,  Littorina  will  react 
to  small  objects  in  its  neighborhood,  whereas  in  an  extreme 


Spatially  Determined  Reactions  223 

state  of  "hydratation"  or  desiccation  it  responds  to  the 
attraction  or  repulsion  of  the  larger  screens  with  fatal 
uniformity  (80). 

Plateau  attempted  to  test  the  responses  of  certain  Dip- 
tera  to  the  size  of  an  opening  admitting  light,  by  placing 
them  in  a  dark  room,  into  which  light  entered  from  two 
sources.  One  was  a  single  orifice  large  enough  to  let  the 
insects  out ;  the  other  was  covered  with  a  net  whose  meshes 
were  too  fine  to  allow  them  to  pass.  The  amount  of  light 
from  the  two  sources  could  be  made  equal.  When  this 
was  done,  the  insects,  which  were  positively  phototropic, 
sought  the  two  equally  often ;  if  the  light  from  either  was 
made  more  intense,  they  went  to  that  one.  Plateau  con- 
cluded both  that  the  flies  could  not  see  the  netting  and  that 
the  area  of  the  light  source  did  not  affect  them  (592). 
On  the  other  hand,  Parker  found  that  the  mourning-cloak 
butterfly  did  discriminate  areas,  flying  to  the  larger  of  two 
sources  of  equally  intense  light  (537). 

This  method  of  testing  the  image-forming  power  of  an 
animal's  eyes  has  been  elaborated  by  L.  J.  Cole.  He  sub- 
jected animals  with  decided  positive  or  negative  photot- 
ropism  to  the  influence  of  two  lights  made  equally  intense 
but  differing  in  area,  one  coming  through  a  piece  of  ground 
glass  41  cm.  square,  the  other  a  mere  point.  Eyeless 
animals,  the  earthworm,  for  example,  reacted  equally 
often  to  each  light.  Animals  whose  eyes  from  their  struc- 
ture have  been  judged  capable  of  perceiving  merely  the 
direction  of  light  rays,  such  as  the  planarian  Bipalium, 
confirmed  the  argument  from  structure  by  showing  little 
more  discrimination  than  the  eyeless  ones.  On  the  other 
hand,  animals  with  well-developed  compound  or  camera 
eyes,  for  example  certain  insects  and  frogs,  did  distinguish 
between  the  lights,  going,  if  positively  phototropic,  toward 


224  The  Animal  Mind 

the  one  of  larger  area ;  if  negatively  phototropic,  away  from 
it  (132). 

Turtles  showed  a  remarkable  keenness  of  discrimination 
in  the  study  made  by  Casteel  (119),  in  which  they  were 
offered  the  choice  of  two  compartments  faced  with  card- 
boards carrying  black  lines  on  a  white  ground.  Two  turtles 
learned  to  discriminate  between  vertical  lines  eight  milli- 
meters in  width  and  vertical  lines  two  millimeters  in  width, 
and  one  gifted  animal  learned  to  distinguish,  first,  lines 
eight  millimeters  wide  from  lines  one  millimeter  wide,  then 
between  a  width  of  four  millimeters  and  a  width  of  one 
millimeter,  then  between  four  and  two,  and  finally  between 
three  and  two  millimeters.  Chicks  proved  equal  to  a 
discrimination  between  a  standard  circle  six  centimeters  in 
diameter  and  one  from  one-fourth  to  one-sixth  larger.  The 
relative  brightness  of  the  circles  was  varied  so  that  the  chicks 
could  not  use  this  as  a  basis  for  their  choices  (102).  White 
rats  can  discriminate  circles  thirty  millimeters  in  diameter 
from  circles  fifty  millimeters  in  diameter,  and  squares 
twelve  centimeters  a  side  from  squares  one  centimeter  a 
side  (411).  Discrimination  of  boxes  differing  in  size  but 
alike  in  form,  placed  in  a  row  along  a  board,  food  having 
been  put  in  one,  was  imperfectly  learned  by  two  Macacus 
monkeys ;  the  errors  leaned  in  the  direction  of  taking  the 
larger  vessel  (401).  Raccoons  were  taught  to  distinguish 
perfectly  between  two  cards,  one  6|  X  6|  inches  square 
and  the  other  4^  X  4^ ,  shown  successively.  The  animals 
were  to  climb  on  a  box  for  food  when  the  larger  card  was 
shown  and  to  stay  down  when  the  smaller  one  appeared. 
As  we  shall  see  later,  L.  W.  Cole,  the  experimenter,  thinks 
the  learning  gave  evidence  not  only  of  a  spatial  image,  but 
of  a  memory  image  (134). 

One  apparent  effect  of  size  upon  visual  perception  relates 


Spatially  Determined  Reactions  225 

to  the  distance  at  which  an  object  produces  a  reaction. 
Caterpillars,  for  example,  are  described  as  giving  evidence  of 
seeing  a  slender  rod  extended  toward  them  at  a  distance  of 
about  a  centimeter ;  large  masses  they  reacted  to  at  some- 
what greater  distance  (597).  It  is  highly  doubtful  whether 
this  means  that  the  simple  eye  of  the  caterpillar  could  give 
a  perception  of  two  objects  as  differing  in  size  if  they  were 
equally  distant.  Myriapods,  which  make  very  little  use  of 
sight  and  do  not  perceive  their  prey  until  they  touch  it, 
give  evidence  of  seeing  an  obstacle  having  a  rather  broad 
surface,  the  size  of  a  visiting  card,  at  a  distance  of  about 
10  cm.,  if  it  is  white  and  reflects  much  light,  or  if  it  is  blue ; 
but  not  if  it  is  red. 


§  67.   The  Visual  Perception  of  Form 

The  second  method  of  studying  visual  images  tests 
an  animal's  power  to  discriminate  forms.  Bumblebees 
were  thought  by  Forel  to  evince  a  capacity  to  distinguish 
a  blue  circle  from  a  blue  strip  of  paper  when  they  had 
previously  found  honey  on  a  blue  circle,  even  though  the 
two  had  been  made  to  exchange  places.  They  flew  first 
to  the  place  where  the  blue  circle  had  been,  but  did  not 
alight  upon  the  strip.  Wasps  also,  according  to  Forel, 
distinguished  among  a  disk,  a  cross,  and  a  band  of  white 
paper,  going  first  to  the  form  on  which  they  had  last  found 
honey  (231).  Turner  (726)  reports  the  ability  of  the  honey- 
bee to  distinguish,  in  the  open  air,  among  "  artefacts " 
of  various  forms  (disks,  cornucopias,  and  boxes),  covered 
with  various  patterns  such  as  transverse  and  longitudinal 
stripes,  mottled  surfaces,  and  spotted  surfaces ;  if  the  bee 
had  found  honey  in  an  artefact  of  a  certain  pattern  it 
would  select  that  pattern  from  among  other  patterns  or 
Q 


226  The  Animal  Mind 

plain  colors.  Von  Frisch  (247)  finds  that  bees  can  dis- 
criminate patterns  like  those  of  flowers,  but  fail  with  those 
very  unlike  flower  patterns.  This  evidence,  taken  at  its 
face  value,  indicates  that  the  compound  eye  is  able  to 
furnish  a  fairly  clear  image,  and  not  merely  discrimina- 
tions of  light  direction  and  movement. 

Among  vertebrates,  various  species  of  birds  were  experi- 
mented on  by  the  method  of  placing  cards  carrying  simple 
designs  over  glasses  covered  with  gray  paper,  food  being 
found  always  under  the  same  card.  The  English  sparrow 
and  the  cowbird  both  learned  to  distinguish  a  card  bearing 
three  horizontal  bars  and  one  bearing  a  black  diamond 
from  each  other  and  from  plain  gray  cards.  On  the  other 
hand,  the  sparrow,  curiously  enough,  did  not  succeed  in  dis- 
criminating vessels  of  different  form ;  the  cowbird  was  not 
fully  tested  with  these,  but  gave  some  evidence  of  learning 
(6 10,  611).  Pigeons  were  only  moderately  successful  in 
a  similar  test  (647).  Breed  (101,  102)  and  Bingham  (56) 
investigated  the  form  discriminations  of  the  chick,  using 
the  more  accurate  method  of  offering  a  choice  between 
compartments  illuminated  through  openings  of  different 
forms.  One  out  of  three  of  Breed's  chicks  succeeded  in 
discriminating  between  a  circle  and  a  square :  Bingham's 
chicks  distinguished  between  a  circle  and  a  triangle  when 
the  apex  of  the  triangle  was  on  top,  but  the  discrimination 
broke  down  when  the  triangle  had  its  base  uppermost. 
The  most  careful  work  that  has  been  done  on  the  discrimi- 
nation of  forms  or  patterns  by  animals,  up  to  the  date  of 
publication  of  this  book,  is  that  of  Johnson  (386).  His 
apparatus  allowed  the  presentation  of  two  illuminated 
fields  whose  intensity  could  be  perfectly  controlled,  with 
black  bands  across  them  whose  width  could  be  varied  at 
will.  He  proposed  four  problems :  (i)  the  width  of  stripes 


Spatially  Determined  Reactions  227 

necessary  to  make  a  striped  field  just  distinguishable 
from  a  uniform  field;  (2)  the  just  noticeable  difference 
between  the  width  of  stripes  on  two  fields;  (3)  the  just 
noticeable  difference  in  the  direction  of  the  stripes  on  two 
fields;  (4)  the  just  noticeable  difference  in  brightness 
between  two  fields,  one  of  which  has  stripes  of  equal  bright- 
ness, while  the  stripes  on  the  other  are  of  unequal  bright- 
ness. The  chick's  ability  to  distinguish  a  striped  from  a 
plain  field  proved  to  be  about  one-fourth  that  of  a  monkey 
or  human  being ;  when  the  problem  of  distinguishing  be- 
tween striped  fields  whose  stripes  were  of  different  widths 
was  presented,  the  monkey  did  ten  times  as  well  as  the  chick. 
For  differences  in  the  direction  of  stripes,  the  threshold  of 
the  chick  was  between  twenty-five  and  thirty  degrees ;  the 
monkey's  was  between  two  and  five  degrees :  moreover, 
the  monkey  learned  the  discrimination  in  twenty  trials, 
while  the  chick  required  585.  It  seems  practically  certain 
that  the  chick  is  not  a  fair  representative  of  the  bird  family 
as  regards  the  clearness  of  its  vision  for  form  and  size; 
the  eye  of  a  hawk  is  a  proverb  for  keenness,  and  the  ability 
of  birds  to  find  their  food  by  vision  demonstrates  the  high 
development  of  their  eyes  in  image-forming  power. 

Among  mammals,  many  dogs  have  been  taught  to  dis- 
tinguish printed  letters  on  cards ;  Sir  John  Lubbock's 
poodle  "Van"  is  a  familiar  example.  Van  learned  to  pick 
out  cards  marked  " Food, "  "Bone, "  "  Out,"  "Water, "  and 
the  like,  and  to  present  each  on  its  appropriate  occasion. 
It  took  him  ten  days  to  begin  to  make  the  first  step  of  dis- 
tinguishing between  a  printed  card  and  a  plain  one;  in 
a  month  this  was  perfected  and  in  twelve  more  days,  when 
he  wanted  food  or  tea,  he  brought  the  right  card  one  hundred 
and  eleven  times  and  the  wrong  one  twice.  The  second 
mistake  consisted  in  bringing  the  word  "Door"  instead  of 


228  The  Animal  Mind 

"Food,"  which  was  taken  as  indicating  that  he  really  was 
paying  attention  to  the  look  of  the  words  (444).  Such 
observations,  however,  are  very  inconclusive  when  com- 
pared with  modern  experimental  studies  where  all  the 
sources  of  error,  from  smell,  for  example,  are  carefully 
controlled.  In  Johnson's  (386)  study  of  the  visual  acuity  of 
the  dog,  while  two  chickens  and  a  monkey  learned  to  dis- 
tinguish a  striped  from  a  plain  field  in  from  three  hundred 
to  four  hundred  trials,  dogs  failed  to  learn  in  over  a  thousand 
trials,  although  the  stripes  were  made  nearly  six  times  as 
wide.  The  dog  could  not  distinguish  between  two  visual 
fields  unless  they  differed  in  intensity.  Thus  his  visual 
images  would  seem  to  be  far  from  clear.  The  eye  of  the 
dog,  it  may  be  noted,  does  not  possess  a  fovea.  Johnson 
thinks  the  dog's  vision  is  useful  chiefly  for  the  perception 
of  moving  objects.  Szymanski  (702)  finds  that  when 
dogs  and  cats  have  been  trained  to  go  to  a  box  in  a  certain 
corner  to  get  food,  and  the  box  is  moved,  the  dogs  show 
their  lack  of  dependence  on  vision  by  displaying  little 
tendency,  as  compared  with  the  cats,  to  use  this  sense  in  find- 
ing the  new  situation  of  the  box.  Orbeli,  however,  obtained 
evidence  by  Pawlow's  method  that  dogs  could  appreciate 
form  and  size  differences  (532  a). 

The  dancing  mouse  could  not  learn  to  distinguish  two 
equal  illuminated  areas  of  different  forms  (820).  Rac- 
coons learned  to  discriminate  a  round  card  from  a  square 
one  (134).  Thorndike  taught  the  two  Cebus  monkeys 
under  his  observation  to  come  down  to  the  bottom  of  the 
cage  for  food  when  a  card  bearing  the  word  "Yes"  printed 
on  it  was  exposed,  and  to  stay  up  when  one  bearing  the 
letter  "N"  was  shown.  The  conditions  seem  to  have 
been  complicated,  however,  by  the  fact  that  the  two  cards 
were  not  placed  in  quite  the  same  position.  Further 


Spatially  Determined  Reactions  229 

tests  with  cards  carrying  various  designs  showed  varying 
degrees  of  capacity  to  distinguish  them  on  the  part  of  the 
monkeys  (708).  Kinnaman  got  negative  results  with 
his  two  Macacus  monkeys  in  attempting  to  train  them  to 
distinguish  cards  such  as  those  used  in  the  later  experi- 
ments of  Porter  on  birds.  His  monkeys,  however,  proved 
able  to  distinguish  vessels  of  different  forms,  "a  wide- 
mouthed  bottle,  a  small  cylindrical  glass,  an  elliptical  tin 
box,  a  triangular  paper  box,  a  rectangular  paper  box,  and 
a  tall  cylindrical  can."  These  vessels  differed  in  size  as 
well  as  in  form  (401).  Johnson's  far  more  accurate  experi- 
ments with  the  striped  fields  give  the  monkey  a  visual 
acuity  about  equal  to  that  of  man. 

The  question  has  been  raised  as  to  just  what  is  meant 
by  the  term  "form"  in  connection  with  the  visual  percep- 
tions of  an  animal.  When  Bingham  (56,  57)  found  that 
a  chick  failed  to  recognize  a  triangle  whose  base  instead 
of  its  apex  was  uppermost,  he  suggested  that  the  chick's 
previous  discrimination  of  the  triangle  from  a  circle  was 
not  a  discrimination  of  form  in  the  true  sense  of  the  word, 
but  based  "on  the  unequal  stimulation  of  different  parts  of 
the  retina."  Hunter  (352)  thinks  that  the  animal  in  such 
a  case  is  really  discriminating  pattern  rather  than  form,  and 
by  pattern  he  means  the  whole  design  presented  by  the 
lighted  forms  and  their  surroundings.  That  is,  a  square 
lighted  area  inside  a  round  tunnel  would  present  to  the 
animal  a  different  pattern  from  a  square  lighted  area 
inside  a  square  tunnel ;  an  animal  might  fail  to  recognize 
that  the  forms  of  the  squares  were  identical  when  they 
were  presented  as  parts  of  such  different  patterns.  The 
writer  of  this  book  suggested  in  a  review  of  Bingham's 
work 1  that  his  chicks,  in  failing  to  recognize  that  a  triangle 
1  Psych.  Bull,  vol.  10  (1913),  p.  320. 


230  The  Animal  Mind 

with  apex  down  is  the  same  form  as  a  triangle  with  apex 
up,  were  demonstrating  not  their  deficiency  in  form  vision, 
but  their  lack  of  an  abstract  idea  of  triangularity.  It 
may  well  be  that  such  a  perception  of  form  in  the  abstract, 
such  an  ability  to  analyze  forms  out  of  patterns,  depends 
upon  the  association  between  visual  impressions  and 
movements  like  the  hand  movements  of  a  human  being; 
few  lower  animals,  of  course,  have  the  same  kind  of  motor 
experience  of  objects  that  man  possesses. 

Special  evidence  of  the  comparative  development  of  the 
visual  image  in  different  genera  of  ants  is  suggested  by 
Wasmann  to  be  furnished  by  the  facts  of  mimicry.  Certain 
insects  belonging  to  orders  other  than  the  Hymenoptera 
inhabit  ants'  nests,  and  have  in  many  cases  become  more 
or  less  modified  to  resemble  their  hosts.  Wasmann  thinks 
that  these  resemblances,  which  have  been  established  on  ac- 
count of  their  protective  value,  are  in  insects  living  among 
ants  of  well-developed  visual  powers,  such  as  would  deceive 
especially  the  sense  of  sight,  while  in  the  "guests"  of  ants 
whose  vision  is  poor,  the  mimicry  is  adapted  to  produce 
tactile  illusions  (762). 

§  68.   The  Homing  of  Animals  as  Evidence  of  Image  Vision 

The  ability  to  find  their  way  back  to  their  dwelling  place, 
or  to  any  other  locality  that  has  a  vital  significance  for 
them,  is  a  power  widely  distributed  among  the  most  vari- 
ous forms  of  animals.  We  have  considered,  in  the  chapter 
on  the  Chemical  Sense,  the  part  which  smell  plays  in  this 
process,  and  on  page  100  we  noted  the  fact  that  the  percep- 
tion of  light  direction  is  not  wholly  without  influence  in 
some  cases.  The  common  human  method  of  path-find- 
ing is  by  the  recognition  of  visual  landmarks:  when  we 


Spatially  Determined  Reactions  231 

set  out  from  a  familiar  region  into  a  strange  region,  we  fix 
our  attention  on  the  appearance  of  the  surroundings  at 
critical  points  and  turnings,  and  on  the  homeward  journey 
guide  ourselves  by  identifying  these  points  through  vision. 
Where  it  can  be  shown  that  animals  are  influenced  in  their 
homing  journeys  by  the  appearance  of  the  surroundings, 
we  have  evidence  that  their  vision  must  involve  some  per- 
ception of  the  form  and  detail  of  objects.  The  fiddler 
crab  "  remembers "  the  location  of  its  nest,  but  just  what 
the  memory  depends  upon  is  not  clear.  On  one  occasion 
the  observer,  Pearse  (568),  covered  the  nest  with  his  foot; 
the  female  crab  to  which  it  belonged  waited  fifteen  minutes 
until  he  moved  his  foot,  and  then  dashed  for  the  nest  and 
tried  to  reopen  it.  Lubbock's  (441)  demonstration  that  ants 
do  not  use  visual  landmarks  on  frequented  roads  will  be 
recalled  (see  page  97). 

In  the  case  of  bees,  on  the  other  hand,  there  is  a  good  deal 
of  evidence  in  favor  of  the  use  of  visual  landmarks  in  hom- 
ing. It  is  true  that  Bethe  (51)  was  unable  to  note  any 
disturbance  in  the  flight  of  bees  back  to  the  hive  when  he 
altered  the  appearance  of  the  hive,  or  when  a  large  tree 
that  stood  near  the  hive  was  cut  down.  But  in  this  case 
the  bees  had  thoroughly  learned  the  location  of  the  hive 
and  had  probably  ceased  to  need  landmarks  in  its  immediate 
environs.  Lubbock  found  that  bees  from  a  hive  near  the 
seashore,  when  taken  out  on  the  water  and  liberated,  were 
unable  to  find  their  way  home,  although  the  distance  was 
less  than  their  usual  range  of  flight  on  land;  and  he 
ascribes  their  failure  to  the  lack  of  visual  landmarks  to 
guide  them  (441).  Bethe,  who  thinks  bees  are  guided  home 
neither  by  vision  nor  by  smell,  but  by  an  unknown  force 
to  which  they  respond  reflexly,  also  liberated  some  bees 
at  sea  about  1700-2000  metres  from  their  hive,  which  was 


232  The  Animal  Mind 

near  the  foot  of  Vesuvius  and  beside  some  very  tall  and 
conspicuous  trees.  The  bees  failed  to  return,  and  Bethe 
thinks,  if  they  were  guided  by  vision,  the  mountain  and  the 
trees  should  have  aided  them  to  do  so  (53).  It  may  well  be, 
of  course,  that  bees  cannot  see  objects  at  such  a  distance. 
Besides  his  observation  that  changing  the  appearance  of  a 
hive  did  not  disturb  the  bees  in  their  homing  flight,  Bethe 
urges  against  the  visual  memory  hypothesis  an  observa- 
tion on  a  hive  which  had  on  one  side  of  it  a  garden,  and  on 
the  other  side  a  town,  which  he  thinks  the  bees  never  visited, 
as  food  was  to  be  had  in  abundance  in  the  garden.  Yet 
when  liberated  in  the  town  they  flew  back  to  the  hive  with 
an  accuracy  certainly  not  born  of  their  acquaintance  with 
the  locality  (51).  Von  Buttel-Reepen,  however,  doubts 
whether  the  bees  really  never  visited  the  town.  Bethe's 
most  striking  illustration  of  his  unknown  force,  however, 
is  derived  from  his  " box-experiments."  If  a  number  of 
bees  are  carried  in  a  box  some  distance  from  the  hive,  on 
being  liberated  they  fly  straight  up  in  the  air.  Some  of 
them  will  return  to  the  hive,  but  if  the  distance  is  great 
enough,  many  will  drop  back  upon  the  box.  Now  if  the 
box  has  moved  only  a  few  centimeters  away  during  the 
flight  of  the  bees,  they  will  drop  back  to  the  precise  spot 
where  it  was,  and  take  no  notice  of  its  new  location.  If 
they  were  guided  by  vision,  Bethe  urges,  they  could  easily 
see  the  box  (51,  53).  This,  says  von  Buttel-Reepen,  is 
arguing  that  their  visual  memory  must  be  like  ours  if  it 
exists  at  all ;  it  may  be  a  memory,  not  of  the  appearance 
of  the  box,  but  of  its  locality.  He  himself,  repeating 
Bethe's  experiments,  observed  the  bees  on  dropping  back 
after  their  upward  flight,  hunting  not  at  the  place  where 
the  box  had  been,  but  at  a  height  which  was  about  that  of 
their  home  hive  entrance.  He  thinks  that  an  important 


Spatially  Determined  Reaction  233 

feature  of  the  bee's  visual  memory  consists  in  a  power  of 
accurately  estimating  height  above  the  ground.  If  the 
entrance  to  the  hive  be  raised  or  lowered  30  cm.,  all  the 
returning  bees  will  go  to  the  old  place,  and  it  will  be  hours 
and  sometimes  days  before  they  find  the  new  one.  More- 
over, the  same  bees  tend  to  return  to  the  same  corner  of 
the  opening  each  time.  When  a  row  of  hives  had  been 
arranged,  some  with  openings  in  front  and  others  with 
openings  at  the  side,  bees  which  had  been  driven  home  in 
haste  by  a  storm  would  sometimes  try  to  enter  the  wrong 
hive,  but  if  their  home  hive  opened  on  the  side,  they  would 
attempt  to  enter  the  foreign  hive  on  the  corresponding 
side  (115). 

Turner  (723  a)?reports  that  the  burrowing  bees  (Antho- 
phoridae)  use  visual  landmarks  to  identify  the  location  of 
their  nests,  and  are  disturbed  if  the  landmarks  are  altered. 

In  the  solitary  wasps,  although  Fabre  is  inclined  to  as- 
sume a  "special  faculty"  of  homing,  independent  of  visual 
memory,  basing  his  assumption  on  experiments  where 
the  wasps  returned  to  their  nests,  from  which  they  had 
been  transported  in  a  box  to  a  distance  of  three  kilometers 
(218,  Series  I) ;  yet  the  evidence  obtained  by  the  Peck- 
hams  seems  fairly  conclusive  in  favor  of  memory  for  visual 
landmarks.  The  solitary  wasps  have  been  shown  by  the 
observations  of  the  Peckhams  to  depend  upon  sight  for 
the  return  to  the  nest  (572,  573),  and  the  same  conclusion 
is  indicated  for  the  social  wasps  by  Enteman  (206).  The 
Peckhams'  belief  in  the  visual  memory  of  solitary  wasps 
rests  first  upon  the  fact  that  the  wasp,  upon  completing 
her  nest,  always  spends  some  time  in  circling  about  the 
locality,  in  and  out  among  the  plants,  as  if  she  were  making 
a  careful  study  of  the  region.  On  leaving  the  nest  a  second 
time  she  omits  this  process  and  flies  straight  away.  A 


234  The  Animal  Mind 

similar  "locality  survey"  is  made  by  hive  bees  and  by 
social  wasps.  Secondly,  the  Peckhams  argue  that  if  the 
wasp  does  not  remember  her  nest  by  landmarks,  it  ought 
to  make  no  difference  to  her  when  the  surroundings  are 
altered  in  any  way.  They  found,  however,  that  a  wasp 
of  one  species  could  not  discover  her  nest  when  a  leaf  that 
covered  it  was  broken  off,  but  found  it  again  without 
trouble  when  the  leaf  was  replaced.  Another  wasp  aban- 
doned the  nest  she  had  made  for  herself  with  much  labor, 
because  the  Peckhams,  to  identify  the  spot  themselves, 
drew  radiating  lines  from  it  in  the  dust.  A  third  argument 
against  the  existence  of  a  special  sense  of  direction  is  the 
fact  that  wasps  sometimes  are  unable  to  find  their  nests. 
In  one  case  the  Peckhams  dug  up  the  nest  of  a  wasp  and 
she  made  another  five  inches  away.  After  an  absence 
of  three  hours  the  wasp  returned,  and  seemed  to  be  puzzled 
as  to  whether  the  old  spot  or  the  new  one  were  the  place 
of  her  nest.  "At  first  she  alighted  upon  the  first  site  and 
scratched  away  a  little  earth,  and  then  explored  several 
other  places,  working  about  for  twelve  minutes,  when  she 
at  last  found  the  right  spot."  Similarly,  when  a  wasp 
that  was  carrying  her  prey  left  it  for  a  few  moments  to  go 
to  the  nest,  as  many  of  them  do,  apparently  to  see  that 
all  is  right  there,  if  any  of  the  surrounding  objects  were 
altered  she  often  had  great  difficulty  in  finding  the  prey 
again.  On  one  occasion  a  wasp  of  another  species  dug  its 
nest  in  the  midst  of  a  group  of  nests  of  the  Bembex  wasp. 
These  latter  are  usually  dug  in  a  wide  bare  space  of  earth 
which  has  no  vegetable  growth  to  serve  as  a  landmark. 
When  the  intruder  had  finished  her  nest,  it  looked  just  like 
the  Bembex  holes.  She  went  away,  secured  a  spider,  and 
when  she  returned  she  could  not  find  her  nest.  "She  flew, 
she  ran,  she  scurried  here  and  there,  but  she  had  utterly 


Spatially  Determined  Reaction  235 

lost  track  of  it.  She  approached  it  several  times,  but  there 
are  no  landmarks  on  the  B.  field.  After  five  minutes  our 
wasp  flew  back  to  look  at  her  spider,"  which  she  had  dropped 
about  three  feet  away,  "and  then  returned  to  her  search. 
She  now  began  to  run  into  the  B.  holes,  but  soon  came 
out  again,  even  when  not  chased  out  by  the  proprietor. 
Suddenly  it  seemed  to  strike  her  that  this  was  going  to 
be  a  prolonged  affair,  and  that  her  treasure  was  exposed 
to  danger,  and  hurrying  back  she  dragged  it  into  the  grass 
at  the  edge  of  the  field,  where  it  was  hidden.  Again  she 
resumed  the  hunt,  flying  wildly  now  all  over  the  field, 
running  into  wrong  holes  and  even  kicking  out  earth  as 
though  she  thought  of  appropriating  them,  but  soon  pass- 
ing on.  Once  more  she  became  anxious  about  the  spider, 
and,  carrying  it  up  on  to  a  plant,  suspended  it  there.  Now 
she  seemed  determined  to  take  possession  of  every  hole 
that  she  went  into,  digging  quite  persistently  in  each, 
but  then  giving  it  up.  One  in  particular  that  was  close  by 
the  spider  seemed  to  attract  her,  and  she  worked  at  it  so 
long  that  we  thought  she  had  adopted  it,  for  it  seemed  to 
be  unoccupied.  At  last,  however,  she  made  up  her  mind 
that  all  further  search  was  hopeless,  and  that  she  had 
better  begin  de  novo;  and  forty  minutes  from  the  time  that 
we  saw  her  first  she  started  a  new  nest  close  to  the  spider,  as 
though  she  would  run  no  more  risks"  (572).  An  occur- 
rence of  this  kind  certainly  lends  color  to  the  '  recognition 
of  landmarks'  theory.  On  the  other  hand,  the  Bembex 
wasps  themselves  find  their  nests  with  unerring  accuracy, 
though  there  is  no  landmark  in  the  field.  Fabre  noted 
that  Bembex  wasps  could  not  be  led  astray  by  any  modi- 
fication of  either  the  look  or  the  smell  of  their  nests,  and 
thought  a  peculiar  form  of  space  memory,  unparalleled 
in  our  own  experience,  must  be  involved  in  the  nest-find- 


236  The  Animal  Mind 

ing  of  this  species  (216,  Series  I,  263).  Bouvier,  repeat- 
ing Fabre's  experiments  on  Bembex,  obtained  a  different 
result.  When  a  stone,  for  example,  that  had  been  at  the 
mouth  of  a  Bembex  nest  was  moved  a  distance  of  2  dm., 
the  wasp,  returning,  went  to  the  stone.  Bouvier  accord- 
ingly maintains  the  visual  landmark  hypothesis  (99). 
Ferton  holds  the  same  view  with  regard  to  a  species  of 
wasp  that  makes  its  nest  in  shells.  If  during  successive 
absences  on  the  wasp's  part  the  shell  is  moved  from  posi- 
tion A  to  position  B,  and  later  from  B  to  C  and  from  C  to 
D,  the  wasp,  returning,  goes  in  turn  to  each  of  the  posi- 
tions that  the  shell  has  occupied.  "In  time,  she  omits  to 
go  to  A,  then  to  B.  Little  by  little,  the  image  of  the  pre- 
vious locations  of  her  nest  is  effaced  in  the  insect's  memory." 
When  she  has  found  it,  after  each  displacement,  she  makes 
a  new  "  locality  survey,"  before  starting  off  again  (217). 

Turner  (728)  reports  that  the  mason  wasp  is  certainly 
guided  by  visual  landmarks.  A  wasp  had  built  her  nest 
on  a  window  casing.  The  window  was  one  of  four  in  a 
row;  the  shades  on  the  other  three  were  down.  When 
the  shade  on  the  window  where  the  wasp's  nest  was  situ- 
ated was  drawn  down  and  that  of  the  next  window  drawn 
up,  the  wasp  returning  sought  her  nest  on  the  casing  of 
the  next  window,  which  was  now  the  only  light  one  in  the 
row. 

Solitary  wasps  and  bees,  which  need  to  find  their  way 
back,  not  to  a  nest  whose  position  remains  fixed,  as  is  the 
case  with  ants  and  honey  bees,  but  to  nests  in  new  positions 
from  day  to  day,  almost  certainly  have  to  depend  upon 
their  recognition  of  visual  landmarks,  and  hence  we  have 
another  evidence  that  the  compound  eye  can  give  a  ser- 
viceable image. 

The  migration  of  birds  is  still  an  unsolved  problem. 


Spatially  Determined  Reaction  237 

That  carrier  pigeons  depend  on  visual  landmarks  is  main- 
tained by  many  authorities.  They  do  not  fly  at  night, 
nor  do  they  home  well  in  cloudy  weather.  Young  pigeons 
have  to  be  trained  on  short  distance  flights,  though  of 
course  this  might  be  the  case  if  they  depended  on  some 
other  power  than  recognition  of  visual  landmarks.  Mi- 
grating birds  in  some  cases  fly  long  distances  over  the  ocean, 
where  no  visual  clues  can  be  furnished.  Watson  (769) 
caused  some  noddy  and  sooty  terns  to  be  carried  in  a  steamer 
from  the  Tortugas  Islands  to  the  latitude  of  Cape  Hatteras, 
a  distance  of  nearly  a  thousand  miles,  where  they  were 
liberated.  The  locality  is  far  out  of  their  range  of  habitat, 
yet  they  returned  to  their  breeding  place  in  about  a  week. 
Hachet-Souplet 1  suggests  that  a  very  vague  visual  image, 
of  objects  too  far  off  to  be  clearly  seen,  may  be  used 
in  such  long  distance  homing,  but  the  curvature  of  the 
earth  would  interfere  with  a  bird's  getting  even  a  vague 
image  of  any  surroundings  that  could  be  familiar  to  it. 

§  69.   Class  V:  Reactions  adapted  to  the  Distance  of  Objects 

The  factors  that  make  possible  the  perception  of  the 
third  dimension,  depth,  or  distance  outward  from  the  body, 
in  invertebrate  animals  are  little  known.  Certain  inverte- 
brates do  give  evidence  of  the  power  to  judge  distance. 
The  hunting  spiders,  for  example,  which  do  not  make  webs, 
but  pursue  their  prey  in  the  open,  leap  on  it  from  a  distance 
of  several  inches.  Dahl  thinks  their  distinct  vision  is  limited 
to  two  centimeters  (168),  and  Plateau  says  capture  is 
not  attempted  until  the  prey  is  within  this  distance  (596). 
The  Peckhams,  however,  tested  a  hunting  spider  by  putting 

1  VI  CongrSs  Int.  de  Psychologic,  1909,  p.  663.  I  have  been  unable  to 
obtain  the  original  article. 


238  The  Animal  Mind 

it  at  one  end  of  a  narrow  glass  case  sixteen  inches  long,  at 
the  other  end  of  which  a  grasshopper  was  placed.  When 
eight  inches  from  its  victim,  the  spider's  movements 
changed,  and  at  four  inches  the  leap  was  made  *  (571). 

Reactions  of  this  character,  where  the  animal  makes  a 
single  movement  adapted  to  the  distance  of  an  object 
from  it,  are  almost  the  sole  evidence  we  can  get  of  accurate 
perception  of  the  third  dimension.  The  alleged  perform- 
ance of  the  jaculator  fish,  which,  as  described  by  Romanes, 
"  shoots  its  prey  by  means  of  a  drop  of  water  projected  from 
the  mouth  with  considerable  force  and  unerring  aim/' 
the  prey  being  "some  small  object,  such  as  a  fly,  at  rest 
above  the  surface  of  the  water,  so  that  when  suddenly 
hit  it  falls  into  the  water,"  would  involve  distance  per- 
ception (640,  p.  248).  The  catching  of  insects  on  the  wing 
by  various  amphibians,  reptiles,  and  birds  has  the  same 
significance.  A  salamander  cautiously  stalking  a  small 
fly  will  not  strike  until  it  gets  within  a  certain  distance. 
In  Necturus  and  in  other  animals  the  pause  just  before 
snapping  at  food  has  been  suggested  to  be  for  the  purpose 
of  proper  fixation  (785). 

Training  an  animal  to  jump  from  one  support  to  another 
is  a  method  that  has  been  used  to  study  distance  percep- 
tion in  the  mouse  (775)  and  white  rat  (634).  Waugh  put 
a  mouse  on  a  disk  and  raised  it  a  certain  distance  above 
a  support ;  he  then  measured  the  time  the  mouse  hesitated 
before  jumping,  when  the  height  of  the  disk  was  varied. 
From  the  fact  that  the  mice  hesitated  longer,  the  greater 
the  height,  he  inferred  some  visual  perception  of  distance. 

1  Porter  observed  that  the  distance  at  which  spiders  of  the  genera  Argiope 
and  Epeira  could  apparently  see  objects  was  increased  six  or  eight  times  if 
the  spider  was  previously  disturbed  by  shaking  her  web  (612).  This,  of 
course,  does  not  refer  to  the  power  to  judge  distance. 


Spatially  Determined  Reaction  239 

When,  however,  the  mice  were  required  to  judge  which  of 
two  partitions  was  nearer  to  their  starting-point,  and  to 
turn  to  the  right  or  the  left  in  accordance  with  this  pre- 
liminary judgment  in  returning  to  their  nest,  they  failed : 
this  really  involves  a  rather  complex  type  of  learning,  and 
is  a  much  less  fair  test  of  the  mere  ability  to  perceive  dis- 
tance than  is  the  instinctive  reaction  of  jumping.  In 
Richardson's  (634)  study  of  the  rat,  the  animals  were 
trained  to  jump  from  one  horizontal  support  to  another. 
They  proved  able  to  judge  quite  accurately  the  direction 
of  the  platform  to  which  they  had  to  jump,  but  when  its 
distance  was  altered  they  could  not  adapt  themselves, 
and  jumped  either  too  far  or  too  short. 

Yerkes's  tests  of  the  so-called  ' sense  of  support'  in  tor- 
toises indicate,  like  Waugh's  experiments  on  the  mouse, 
some  power  of  estimating  distance  by  vision  in  these  ani- 
mals. He  experimented,  it  will  be  remembered,  with 
individuals  belonging  to  three  classes :  land-dwelling, 
water-dwelling,  and  amphibious.  The  first  mentioned 
would  crawl  off  the  edge  of  a  board  30  centimeters  above 
a  net  of  black  cloth  only  with  much  reluctance  when  their 
eyes  were  uncovered;  when  blindfolded  they  would  not 
move  at  all.  The  water  tortoises  plunged  off  without 
hesitation  from  a  height  of  30  centimeters,  but  hesitated 
slightly  at  90  centimeters,  although  some  individuals 
would  take  the  plunge  at  once  even  from  a  height  of  180 
centimeters.  When  blindfolded,  all  of  the  water  tortoises 
rushed  off  at  any  height.  The  land-and-water-dwelling 
tortoises  hesitated  at  30  centimeters  and  at  90  centimeters 
showed  a  conflict  of  impulses,  trying  to  catch  themselves 
before  launching  off.  When  blindfolded  they  would  not 
leave  the  board  at  all,  though  they  moved  about  upon  it 
freely  (810). 


240  The  Animal  Mind 

Some  of  the  most  important  conditions  of  distance  per- 
ception in  our  own  experience  are  lacking  in  the  lower 
vertebrates  and  in  invertebrates.  Stereoscopic  vision,  the 
appearance  of  solidity  given  to  objects  by  the  fact  that  the 
visual  fields  of  the  two  eyes  combine,  thus  producing  blend- 
ing of  two  slightly  different  views  of  the  object  looked  at, 
has  been  held  to  be  dependent  on  the  partial  crossing  of 
the  optic  nerves  on  their  way  to  the  brain,  whereby  each 
retina  sends  nerve  fibres  to  both  hemispheres  of  the  brain. 
This  arrangement  does  not  appear  in  the  animal  kingdom 
below  the  birds;  whatever  function  it  plays  in  space 
perception  is,  then,  absent  from  reptiles,  amphibians,  fish, 
and  invertebrates.  Certainly  stereoscopic  vision  cannot 
exist  in  animals  whose  eyes  are  so  placed  that  the  same 
object  cannot  be  seen  by  both,  as  is  the  case  with  most 
fishes.  In  birds  whose  eyes  are  situated  too  far  toward 
the  sides  of  the  head  for  the  same  object  to  cast  its  images 
on  the  foveas  or  centres  of  the  two  retinas,  there  appears 
to  be  a  secondary  fovea  in  each  eye,  so  placed  as  to  suggest 
that  it  serves  binocular  vision,  while  the  primary  fovea  is 
used  for  monocular  vision.  In  certain  mammals  the  eyes 
are  placed  so  far  towards  the  sides  of  the  head  that  the 
binocular  field  is  very  small.  This  is  probably  the  reason 
why  rodents  do  not  have  a  more  accurate  perception  of  dis- 
tance. The  writer  made  some  simple  tests  on  the  use  of 
binocular  and  monocular  vision  by  the  rabbit  (756). 
When  the  animal  was  sitting  quietly,  two  bits  of  food 
of  equal  size  and  kind  were  held  at  equal  distances  from 
the  rabbit's  nose,  one  straight  in  front,  the  other  directly 
to  the  right  or  left  of  the  rabbit's  head.  In  forty-eight 
out  of  fifty  trials,  the  rabbit  turned  towards  and  secured 
the  food  at  the  side  rather  than  that  in  front,  thus  showing 
its  dependence  on  monocular  rather  than  binocular  vision. 


Spatially  Determined  Reaction  241 

Convergence,  the  turning  of  the  eyes  toward  each  other 
to  bring  the  two  images  of  an  object  on  the  central  part 
of  the  retinas,  which  is  an  important  aid  to  human  estima- 
tions of  distance,  is  also  necessarily  lacking  in  animals 
without  binocular  vision.  A  third  factor  in  our  own  per- 
ceptions of  distance,  the  accommodation  of  the  crystalline 
lens,  that  is,  the  alteration  of  its  convexity  through  the 
pull  of  the  accommodation  muscle  to  enable  it  to  focus 
objects  at  different  distances,  has  been  carefully  studied 
in  connection  with  the  lower  animals  by  Beer.  Through 
experiments  on  the  refractive  powers  of  eyes  dissected 
from  the  dead  animal,  he  reached  the  conclusion  that  no 
invertebrates  but  cephalopods  have  the  power  of  accom- 
modation. It  is  rudimentary  or  lacking  also  in  some  mem- 
bers of  the  fish,  lizard,  crocodile,  snake,  and  mammal 
families.  In  cephalopods,  fishes,  amphibians,  and  most 
reptiles,  the  process  of  accommodation  does  not  involve  a 
change  in  the  form  of  the  lens,  but  an  alteration  in  the  dis- 
tance between  the  lens  and  the  retina.  The  device  of 
increasing  the  curvature  of  the  lens  for  vision  of  near 
objects  appears  first  in  certain  snakes,  and  is  found  through- 
out the  higher  vertebrates  (33,  34,  35,  37). 

Where  accommodation  does  not  exist,  as  in  most  in- 
vertebrates, it  is  possible  to  trace  other  arrangements  for 
adapting  vision  to  the  distance  of  the  object  seen.  Thus 
in  compound  eyes,  part  of  the  eye  may  be  adapted  to  near 
vision  and  part  to  far  vision.  This  is  suggested  by  the 
fact  that  some  of  the  little  tubes,  or  ommatidea,  of  which 
the  compound  eye  is  composed,  diverge  from  each  other 
by  a  less  angle  than  others,  indicating  that  they  are  suited 
to  the  reception  of  more  nearly  parallel  rays.  In  insects 
with  both  simple  and  compound  eyes  one  form  may  be 
used  for  near  and  one  for  far  vision.  It  has  been  main- 


242  The  Animal  Mind 

tained  (182)  that  the  simple  eyes  function  with  the  com- 
pound eyes  to  respond  to  changes  in  the  depth  of  objects, 
since  such  changes  would  alter  the  angle  at  which  light 
rays  from  the  object  would  fall  on  the  two  sets  of  eyes. 
Spiders  appear  to  have  the  principal  eyes  adapted  for 
far  vision  and  the  auxiliary  eyes  for  near  vision,  while 
one  spider,  Epeira,  has  part  of  the  hinder  median  eye 
adapted  to  each  (324). 

§  70.   Some  Theoretical  Considerations 

The  temptation  is  strong  to  speculate  upon  the  essential 
nature  of  the  conditions  which  make  possible  true  space 
perception,  the  simultaneous  experiencing  of  sensations 
that  are  referred  to  different  points  in  space.  Such  specu- 
lation must  be  of  the  most  tentative  description,  yet  the 
following  suggestions  seem  not  wholly  unwarranted  by 
the  facts.  For  one  thing,  it  looks  probable  that  the  ability 
to  suspend  immediate  reaction  is  essential  to  space  per- 
ception. Can  a  spatial  complex  of  sensations  occur  in 
the  experience  of  an  organism  unless  that  organism  is 
capable  of  receiving  a  number  of  stimuli  on  a  sensitive 
surface  and  of  suspending,  for  a  brief  period  at  least,  all 
reaction  ?  Let  us  take  as  an  example  of  such  a  complex 
a  visual  field,  within  which  different  color  and  brightness 
qualities  are  arranged  in  definite  order,  some  above,  some 
below,  some  to  the  right,  others  to  the  left.  Could  such 
a  balance  of  tendencies  to  move  the  eye  as  is  involved  in 
the  simultaneous  perception  of  a  number  of  elements  pre- 
serving regular  space  relations  to  each  other  have  been 
brought  about  unless  no  single  one  of  the  tendencies  were 
irresistible?  One  can  readily  imagine  an  eye  functioning 
in  such  a  way  that  every  stimulation  of  it,  though  occa- 


Spatially  Determined  Reaction  243 

sioned  by  rays  from  several  different  directions  acting 
simultaneously,  should  issue  at  once  in  a  resultant  move- 
ment. Would  not  the  accompanying  consciousness  be 
a  single  resultant  sensation,  rather  than  a  complex  of 
spatially  ordered  elements?  It  is  a  good  deal  easier,  of 
course,  to  ask  than  to  answer  such  questions. 

Again,  the  power  of  getting  true  spatial  images  seems  to 
be  bound  up  closely  with  the  power  of  moving  the  sensitive 
surface.  We  get  our  best  tactile  space  perceptions  through 
active  touch,  involving  movement  of  the  hands  and  fingers ; 
our  visual  space  perceptions  are  profoundly  influenced  by 
eye  movements.  Where  the  movements  of  an  animal's 
body  as  a  whole  are  very  rapid,  as  in  the  case  of  winged 
insects,  this  fact  may  compensate  for  the  immovability 
of  its  eyes.  Forel,  as  we  have  seen,  thinks  that  insects 
which  can  explore  objects  by  moving  the  antennae,  bearing 
the  organs  of  smell,  over  them,  may  have  smell  space  per- 
ceptions, such  as  are  unknown  to  our  experience ;  they  may 
perceive  the  shape  and  size  of  odorous  patches  as  we  could 
do  if  our  organs  of  smell  were  on  our  hands  (233).  Now, 
movement  of  a  sense  organ  brings  about  the  same  result 
that  movement  of  a  stimulus  across  a  resting  sense  organ 
does ;  that  is,  the  stimulus  affects  different  points  of  the 
sensitive  surface  in  succession.  But  the  vital  significance 
of  the  two  is  quite  different ;  movement  of  an  object  across 
a  resting  sense  organ  means  very  likely  that  the  object  is 
alive;  it  must  be  instantly  reacted  to,  and  the  speed  of 
the  reaction  is  unfavorable  to  the  formation  of  a  true 
space  perception.  Movement  of  the  sense  organ,  however, 
gives  a  series  of  impressions  on  successive  points  of  the  sensi- 
tive surface,  from  a  resting  object.  While  the  sense  organ 
is  being  moved,  it  is  probable  that  other  reactions  of  the 
animal  will  be  suspended.  Whether  any  part  in  the  forma- 


244  The  Animal  Mind 

tion  of  that  complex  conscious  content  which  we  call  a 
spatial  image,  consisting  of  different  sensations  simul- 
taneously apprehended,  is  played  by  the  "  las  ting  over" 
of  the  impressions  on  one  sensitive  point  after  the  stimulus 
has  passed  on  to  the  next,  a  phenomenon  which  we  find 
both  in  touch  and  in  sight  sensations,  it  is  impossible  to 
say.  We  are,  however,  apparently  justified  in  the  state- 
ments that  the  essence  of  space  perception,  as  distinct  from 
other  conscious  processes  that  may  accompany  spatially 
determined  reactions,  is  the  presence  of  an  image  in  the 
sense  above  defined,  and  that  a  movable  sense  organ  is 
an  important  condition  for  the  production  of  such  an  image. 


CHAPTER  X 

THE   MODIFICATION   OF   CONSCIOUS   PROCESSES   BY 
INDIVIDUAL  EXPERIENCE 

THE  reactions  of  animals  to  stimulation  show,  as  we  re- 
view the  various  animal  forms  from  the  lowest  to  the  highest, 
increasing  adaptation  to  the  qualitative  differences  and 
to  the  spatial  characteristics  of  the  stimuli  acting  upon 
them.  It  is  therefore  possible  to  suppose  that  the  animal 
mind  shows  increasing  variety  in  its  sensation  contents, 
and  increasing  complexity  in  its  spatial  perceptions.  But 
besides  this  advance  in  the  methods  of  responding  to 
present  stimulation,  the  higher  animals  show  in  a  growing 
degree  the  influence  of  past  stimulation.  While  a  low 
animal  may  apparently  react  to  each  stimulus  as  if  no  other 
had  affected  it  in  the  past,  one  somewhat  higher  may  have 
its  reaction  modified  by  the  stimulation  which  it  has  just 
received.  An  animal  still  more  highly  developed  may 
give  evidence  of  being  affected  by  stimuli  whose  action 
occurred  some  time  before ;  and  finally,  in  certain  of  the 
vertebrates,  perhaps,  as  in  man,  conduct  may  be  deter- 
mined by  the  presence  in  consciousness  of  a  memory  idea 
representing  a  past  stimulus.  "Learning  by  experience," 
or  "associative  memory/'  as  we  saw  in  Chapter  II,  has  been 
regarded  as  the  evidence  par  excellence  of  the  existence  of 
mind  in  an  animal.  That  it  does  not  serve  this  purpose 
to  entire  satisfaction  was  also  pointed  out  in  that  earlier 
chapter,  and  will  be  more  clearly  apparent  as  we  survey 

245 


246  The  Animal  Mind 

in  the  following  pages  the  various  ways  in  which  an  organ- 
ism's past  experience  may  modify  its  behavior.  For  each 
type  of  modification  we  shall  try  to  find  a  parallel  in  human 
experience,  and  thus  to  interpret,  so  far  as  possible,  the 
conscious  aspect  of  the  learning  process.  To  begin  with, 
we  shall  distinguish  between  those  modifications  which 
depend  on  some  comparatively  lasting  alteration  in  the 
organism  (in  its  nervous  system  if  it  has  one),  that  is,  the 
kind  of  modification  which  is  ordinarily  understood  by 
the  term  "learning" ;  and  modifications  which  are  due  to 
a  change  essentially  temporary  in  its  character,  in  the 
physiological  state  of  the  organism.  Even  in  the  lowest 
animals  the  effect  of  a  stimulus  depends  on  the  organism's 
physiological  condition,  and  this  condition  is  often  the  re- 
sult of  stimulation  recently  received. 

§  71.  Modifications  Due  to  Essentially  Temporary  Physio- 
logical States :  (a)  Heightened  Reaction  as  the  Result  of  Pre- 
vious Stimulation. 

Sometimes  the  effect  of  the  stimuli  which  the  organism 
has  just  received  increases  the  violence  of  its  response  to 
a  given  stimulus.  Thus  in  the  earthworm  Jennings  points 
out  that  various  stages  of  excitability  may  exist,  due  to 
the  action  of  previous  stimulation,  and  varying  all  the  way 
from  a  state  of  rest,  where  a  slight  stimulus  produces  no 
effect,  to  a  condition  of  violent  excitement,  where  moderate 
stimulation  will  cause  the  animal  to  "whip  around"  into 
a  reversed  position  or  wave  its  head  frantically  in  the  air 
(377).  This  increased  excitability  suggests  the  "nervous 
irritation"  produced  in  a  human  being  by  an  accumu- 
lation of  disagreeable  stimuli ;  an  increased  feeling  of 
unpleasantness  accompanied  by  more  diffused  organic 


Modification  by  Experience  247 

and  kinaesthetic  sensations  is  its  accompaniment  in  the 
human  mind. 

When  the  same  stimulus  is  repeated,  in  many  cases  the 
effect  of  this  heightened  excitability  is  shown  by  the  or- 
ganism's performing  in  succession  different  forms  of  the 
negative  reaction  until  one  of  them  is  successful  in  getting 
rid  of  the  stimulus.  The  ciliate  Stentor  furnishes  us  with 
an  example.  When  attached  by  its  stem,  if  it  is  strongly 
stimulated,  say,  with  a  glass  rod,  several  times  in  succession, 
it  first  tries  its  commonest  negative  reaction,  bending  over 
to  one  side.  If  the  stimulus  continues,  it  reverses  momen- 
tarily the  direction  in  which  the  cilia  are  whirling.  If  this, 
several  times  repeated,  does  not  succeed  in  getting  rid  of 
the  stimulus,  the  animal  contracts  strongly  upon  its  stem. 
This  also  is  continued  for  some  time,  but  if  the  stimulus 
too  is  kept  up,  the  Stentor  finally  breaks  from  its  moorings 
and  swims  off  (370). 

There  are  many  examples  of  similar  behavior  in  other 
animals.  Hydra  in  certain  cases  tries  first  the  ordinary 
negative  response  of  contraction,  and  later  moves  away 
from  the  region  it  has  been  occupying  (751  a).  Frandsen 
found  that  if  the  slug  Limax  maximus  has  a  tentacle  touched 
several  times  in  succession,  it  at  first  withdraws  the  ten- 
tacle and  turns  away  from  the  stimulus.  Later,  it  may 
move  toward  and  push  against  the  stimulus,  and  do  the 
same  if  the  touch  is  on  the  side  of  its  body,  resisting  and 
curving  around  the  obstacle  —  another  way,  of  course,  of 
getting  rid  of  it  (236).  Preyer,  again,  observed  a  very 
pretty  instance  of  this  sort  of  behavior  in  the  starfish.  He 
slipped  a  piece  of  rubber  tubing  over  the  middle  part  of 
one  of  the  arms  of  a  starfish  belonging  to  a  species  in  which 
those  members  are  very  slender,  and  found  that  the  animal 
tried  successively  various  devices  to  get  rid  of  the  foreign 


248  The  Animal  Mind 

body,  to  wit,  the  following :  rubbing  it  off  against  the 
ground,  shaking  it  off  by  holding  the  arm  aloft  and  waving 
it  pendulum-wise  in  the  air,  holding  the  tube  against  the 
ground  with  a  neighboring  arm  and  pulling  the  afflicted 
arm  out,  pressing  other  arms  against  the  tube  and  pushing 
it  off,  and,  finally,  as  a  last  resort,  amputating  the  arm. 
This,  says  Preyer,  is  intelligence,  for  the  emergency  is  not  one 
normal  to  the  animal,  and  it  is  adapting  itself  to  new  con- 
ditions (617).  It  would,  however,  be  demanding  too  much 
even  from  intelligence  to  suppose  that  the  starfish's  behavior 
is  entirely  new.  A  human  being,  capable  of  ideas,  could 
only,  in  a  similar  predicament,  "  think  of,"  that  is,  call  up, 
ideas  of  the  behavior  which  on  former  occasions  somewhat 
resembling  the  present  had  proved  effective.  Do  such 
cases  of  the  trial  of  different  devices  indicate  that  the 
animal  concerned  calls  up  any  kind  of  idea  or  image  of  each 
device  before  putting  it  into  practice?  Decided  evidence 
in  favor  of  such  a  supposition  might  be  furnished  if  the 
"trial  and  error"  needed  to  be  gone  through  with  only 
once.  A  human  being  brought  into  such  conditions  and 
guiding  his  conduct  by  ideas  would,  if  placed  in  a  similar 
emergency  •  soon  afterwards,  immediately  recall  the  idea 
of  the  successful  action  and  waste  no  time  over  the  un- 
successful ones.  But  we  have  no  reason  to  think  that  such 
is  the  fact  with  our  primitive  animals.  Preyer's  starfish, 
when  confined  by  large  flat-headed  pins  driven  into  the 
board  on  which  it  lay,  close  up  in  the  angles  between  its 
arms,  managed  to  escape  by  trying  a  large  variety  of  move- 
ments, and  gradually  diminished,  Preyer  says,  the  num- 
ber of  useless  movements  made  in  successive  experiments 
(617).  O.  C.  Glaser,  on  the  other  hand,  found  that 
the  echinoderm  Ophiura  brevispina  does  not  improve 
at  all  with  practice  in  removing  obstructions  from  its 


Modification  by  Experience  249 

arms.  The  very  versatility  of  the  starfish,  this  writer 
thinks,  tells  against  its  perfecting  any  one  movement 
through  experience  (260).  S  ten  tor  and  Hydra  go  through 
the  same  series  of  reactions  each  time,  without  apparently 
being  influenced  by  their  previous  behavior.  And  again 
we  must  remind  ourselves  that  there  is  no  reason  why  their 
conduct,  adaptively  regarded,  should  be  otherwise.  An 
animal  with  so  little  power  of  distinguishing  qualitative 
differences  among  stimuli  cannot  be  in  any  way  aware 
that  the  stimulus  which  affects  it  a  second  time  is  going, 
as  in  the  previous  case,  to  be  so  persistent  that  the  ordinary 
negative  reaction  will  not  get  rid  of  it.  Further,  each  re- 
action of  the  series  performed  by  the  animal  is  more  dis- 
turbing to  its  ordinary  course  of  life  than  the  preceding  one. 
The  Stentor  can  bend  to  one  side  and  still  continue  the 
food- taking  process ;  if  it  reverses  its  ciliary  action,  feeding 
must  be  momentarily  interrupted;  while  contraction  on 
the  stem  and  breaking  loose  from  its  moorings  are  still 
more  serious  infractions  of  the  normal  routine.  It  would 
be  decidedly  disadvantageous  to  take  the  last  step  while 
there  was  any  chance  that  milder  measures  might  prevail. 
In  all  probability,  since  the  behavior  just  described  has 
no  permanent  effect  upon  the  animal,  it  is  physiologically 
due,  as  Jennings  suggests  (375),  to  the  overflow  of  the  ner- 
vous energy  set  free  by  the  stimulus  into  first  one  channel 
and  then  another.  In  most  cases  the  movements  resulting 
are  all  adapted  to  getting  rid  of  the  stimulus,  though  only 
one  of  them  is  successful  in  so  doing ;  but  we  have  on  record 
one  case  where,  in  a  supreme  emergency,  the  stimulus 
being  not  only  repeated  but  increased  in  intensity,  every 
possible  outlet  is  tried,  whether  it  has  any  fitness  to  the 
situation  or  not.  This  was  observed  by  Mast,  testing  the 
effect  of  increased  temperature  on  the  reactions  of  pla- 


250  The  Animal  Mind 

narians.  The  first  influence  of  such  increase  from  23  de- 
grees to  26  degrees  C.  is  to  produce  heightened  activity 
and  positive  reactions.  Then,  from  26  degrees  to  38  de- 
grees, the  reactions  are  negative.  From  38  degrees  to  39 
degrees,  violent  crawling  movements  set  in,  and  then, 
curiously  enough,  the  righting  reaction  is  given,  perfectly 
irrelevant,  of  course,  to  the  conditions.  Finally,  the  an- 
terior and  posterior  ends  are  turned  under,  the  central 
part  is  arched  upward,  and  the  animal  falls  over  forward 
on  its  back  (462). 

In  all  these  cases  where  repetition  of  the  same  stimulus 
produces  successively  different  forms  of  the  negative  re- 
action increasing  in  violence,  it  is  most  natural  to  think 
of  the  psychic  accompaniment  as  an  increasing  degree  of 
unpleasantness.  In  our  own  experience,  repeating  a  stimu- 
lus does  not  alter  the  quality  of  the  resulting  sensation, 
except  where  the  structure  of  a  special  sense  organ  is  a 
modifying  factor,  as  in  the  case  of  visual  after-images. 
A  decidedly  disagreeable  stimulus  acting  repeatedly  on  a 
human  being  may  produce  unpleasantness  that  grows  more 
and  more  intense  until  it  is  unbearable ;  the  behavior  of  a 
human  being  under  such  circumstances  is  much  like  the 
animal  behavior  we  have  just  been  describing.  Various 
movements  calculated  to  get  rid  of  the  stimulus  are  tried, 
each  more  energetic  than  the  last.  Hence,  if  the  lower 
animals  behaving  thus  are  conscious,  we  may  plausibly 
assert  that  their  consciousness  under  these  circumstances 
is  increasingly  unpleasant.  But  the  human  experience  in 
such  a  case  would  be,  or  might  be,  further  characterized 
by  the  presence  of  ideas.  That  is,  the  human  being 
would  think  of  the  different  ways  to  get  rid  of  the  stimulus 
one  after  another.  This  many,  at  least,  of  the  animals 
that  try  different  negative  reactions  are  apparently  in- 


Modification  by  Experience  251 

capable  of  doing.  We  judge  that  they  are  so  by  the  simple 
fact  that  on  being  subjected  after  an  interval  to  the  same 
presumably  disagreeable  stimulus,  they  do  not  at  once 
make  the  reaction  that  was  previously  successful  in  getting 
rid  of  it.  A  human  being,  recalling  that  reaction  in  idea, 
would  be  able  to  do  so.  We  shall  see  in  the  next  chapter 
that  many  animals,  while  they  do  not  learn  the  successful 
reaction  from  a  single  experience,  do  gradually  diminish 
the  number  of  unsuccessful  ones  made  in  a  series  of  ex- 
periences. It  is  quite  possible  that  this  will  prove  to  be 
true  of  all  animals,  as  experimental  evidence  accumulates. 

§  72.  Modification  Due  to  Essentially  Temporary  Physio- 
logical States :  (b)  Cessation  of  Reaction  to  a  Repeated  Slight 
Stimulus. 

The  type  of  modified  response  just  described  occurs 
when  the  stimulus  is  strong,  and  presumably  injurious. 
When  it  is  of  moderate  intensity  only,  the  organism  tends 
to  respond  less  and  less  violently  as  the  stimulus  is  re- 
peated at  short  intervals,  until  finally  the  response  lapses 
entirely.  The  Ciliata  Vorticella  and  Stentor,  which  spend 
a  part  of  their  time  attached  to  solids  by  a  contractile 
stem,  contract  at  the  first  application  of  a  moderately  in- 
tense mechanical  stimulus,  but  fail  to  react  at  all  when  the 
stimulus  is  several  times  repeated  (370).  Hydra  responds 
to  mechanical  stimulation  by  contraction,  but  gets  used  to 
the  process  when  repeated  and  gives  no  further  reaction 
(751  a).  The  sea-anemone  Aiptasia  reacts  by  a  sharp  con- 
traction to  a  drop  of  water  falling  on  it ;  later  it  ceases  its 
response  to  this  stimulus.  If  exposed  to  light,  it  contracts 
and  remains  in  this  state  for  some  hours,  but  afterwards 
expands  again  (374).  The  annelid  Bispira  wluticornis 


252  The  Animal  Mind 

was  found  by  Hesse  to  give  no  further  response  to  sudden 
shadows  when  the  stimulus  was  frequently  repeated  (321). 
Hargitt  (285)  reports  the  same  of  tube-dwelling  annelids. 
Von  Uexkiill  reports  that  the  sea-urchin  Centrostephanus 
longispinus  ceased  to  respond  to  shadows  after  three  suc- 
cessive stimulations  (736).  Nagel  observed  that  certain 
eyeless  mollusks  which  react  to  sudden  darkening  very 
quickly  get  used  to  the  stimulus  and  cease  to  respond; 
often  after  one  reaction  they  decline  to  react  for  several 
hours.1  The  mollusks  that  responded  to  sudden  bright- 
ening rather  than  to  shadows,  that  were  in  Nagel's  phrase 
photoptic  rather  than  skioptic,  took  longer  to  become  ac- 
customed to  repeated  stimulation,  but  did  so  by  gradually 
weakening  their  reaction  (520).  A  web-making  spider 
that  was  found  by  the  Peckhams  to  drop  from  its  web  at 
the  sound  of  a  large  tuning  fork  declined  to  disturb  itself 
after  the  stimulus  had  been  repeated  from  five  to  seven  times 
(570).  Ants  " become  used"  to  the  ultra-violet  rays  which 
they  ordinarily  avoid  (220).  The  responses  of  dragon  fly 
nymphs  to  light  are  less  marked  as  the  stimulus  is  re- 
peated (636),  and  the  same  is  true  of  mosquito  larvae 

(338). 

Where  such  an  effect  as  this  is  temporary,  the  most 
obviously  suggested  cause  for  it  is  fatigue.  In  our  own 
experience  this  word  is  used  chiefly  with  reference  to  motor 
processes ;  we  perceive  a  certain  signal,  but  are  too  fatigued 
to  respond.  On  the  sensory  side,  when  a  repeated  or  con- 
tinued stimulus  is  no  longer  perceived,  we  call  the  phenom- 

1  The  opposite  phenomenon  is  reported  by  Rawitz  of  the  mollusk  Pecten, 
whose  response  to  a  shadow  was  the  shutting  of  its  shell.  Repeated  or  long- 
continued  shadowing,  instead  of  doing  away  with  the  reaction,  caused  the 
animal  to  remain  with  closed  shell  for  a  long  time ;  an  intensification  of  the 
reaction  which  suggests  the  effect  of  summation  of  stimuli  (628).  We  may 
infer  that  the  stimulus  in  such  a  case  is  injurious. 


Modification  by  Experience  253 

enon  one  of  adaptation.  In  true  sensory  adaptation,  the 
sense  organ  becomes  incapable  of  responding  to  the  stimu- 
lus ;  for  example,  a  person  who  has  been  for  some  time  sub- 
jected to  a  certain  odor  is  unable  to  smell  it  any  more, 
however  much  he  tries.  Closely  related  to  this  phenom- 
enon and  yet  different  from  it,  is  the  lapse  of  attention  to  a 
repeated  stimulus:  we  no  longer  notice  the  ticking  of  a 
clock,  although  the  sense  organ  is  unaffected  by  its  con- 
tinuance, and  we  can  quite  well  hear  it  if  our  attention  is 
attracted  in  that  direction. 

That  the  failure  of  Stentor  to  respond  to  successive  stimuli 
is  not  due  to  motor  fatigue  appears  quite  certain  to  Jen- 
nings, since  under  favorable  conditions  he  has  obtained  re- 
actions from  the  animal  for  a  period  far  longer  than  that 
occupied  by  the  process  of  getting  used  to  slight  mechanical 
stimulation  (370).  And  in  most  of  the  cases  cited,  the 
acclimatizing  process  seems  to  occur  too  rapidly  to  make 
fatigue  of  the  motor  apparatus  probable.  In  the  lower 
animal  forms,  sensory  adaptation  offers  the  most  natural 
explanation  for  the  phenomenon ;  in  the  higher  animals, 
lapse  of  attention  is  very  likely  also  involved.  The  modi- 
fication of  consciousness  in  both  cases  would  be  the  loss 
of  the  sensation;  where  adaptation  occurs,  the  sensation 
would  be  for  the  time  irrecoverably  lost;  where  there  is 
merely  lapse  of  attention,  it  could  be  regained  by  a  proper 
direction  of  attention. 

A  much  discussed  case  of  the  cessation  of  response  to  a 
repeated  stimulus  is  found  in  connection  with  the  food- 
taking  reaction.  One  would  expect  the  dominant  condition 
here  to  be  loss  of  hunger,  and  as  a  matter  of  fact,  observ- 
ers of  the  feeding  processes  in  many  lower  animals  have 
found  that  such  reactions  cease  or  turn  into  negative 
responses  when  the  animal  is  satiated ;  although  Pieron 


254  The  Animal  Mind 

indeed  reports  that  while  the  responses  of  Actinia  equina 
and  A.  rubra  to  mechanical  stimulation  cease  on  repetition 
of  the  stimulus,  those  to  food  stimulation  continue  indefi- 
nitely (581).  If  the  change  from  food-taking  to  negative 
reaction  has  a  conscious  accompaniment,  this  might  natu- 
rally be  thought  of  as  a  change  from  pleasant  to  unpleasant 
affective  tone.  Nagel  observed  that  if  a  ball  of  filter  paper 
soaked  in  fish  juice  were  placed  upon  one  of  the  tentacles 
of  the  sea-anemone  Adamsia,  it  was  seized  as  eagerly  as  a 
ball  of  fish  meat,  but  that  when  this  deception  had  been 
several  times  repeated,  the  ball  was  held  for  a  shorter 
period  each  time,  and  was  finally  rejected  as  soon  as  offered. 
Nagel  is  inclined  to  think  that  this  is  learning  by  experi- 
ence, and  points  out  that  the  psychic  life  of  Adamsia  must 
possess  little  unity,  for  the  "experience"  of  one  tentacle 
does  not  lead  other  tentacles  to  reject  the  paper  balls  at 
once  (521).  Parker  finds  similar  behavior  in  Metridium, 
and  explains  it  by  saying  that  the  filter  paper  offers  but  a 
weak  food  stimulus,  and  that  "the  successive  application 
of  a  very  weak  stimulus  is  accompanied  by  ...  a  gradual 
decline  in  the  effects,  till  finally  the  response  fails  entirely" ; 
in  other  words,  that  we  have  adaptation  to  a  food  stimulus 
(533).  Jennings  fed  Aiptasia  alternately  with  pieces  of 
crab  meat  and  with  filter  paper  soaked  in  meat  juice,  the 
result  being  that  the  fifth  piece  of  filter  paper  was  rejected 
—  but  so  was  the  crab  meat  thereafter.  Jennings  came 
to  the  conclusion  that  the  phenomenon  is  due  simply  to 
loss  of  hunger  on  the  animal's  part,  and  that  where  Parker 
found  that  the  crab  meat  would  be  taken  after  the  filter 
paper  was  refused,  it  was  because  the  latter  was  a  weaker 
stimulus  and  naturally  was  the  first  to  call  forth  the  effects 
of  satiety.  The  objection  to  the  hunger  hypothesis  is  that 
other  tentacles  of  the  same  animal  will  react  after  one 


Modification  by  Experience  255 

tentacle  has  stopped ;  satiety  ought  surely  to  affect  the 
entire  organism  (374).  Allabach,  in  the  light  of  these  re- 
searches, made  a  careful  study  of  Metridium.  She  dis- 
poses of  the  psychic  learning  by  experience  theory  of 
Nagel  by  saying  that  the  only  experience  upon  which  the 
animal  could  reject  the  filter  paper  must  be  experience  that 
it  is  not  good  for  food.  This  could  be  learned  only  by 
swallowing  it;  but  the  failure  of  the  reaction  occurs  just 
as  well  when  the  animal  is  prevented  from  swallowing  the 
filter  paper.  That  the  phenomenon  is  not  one  of  adapta- 
tion to  weak  stimuli  is  shown  by  the  fact  that  it  may  be 
brought  about  by  successive  feedings  with  meat  which  is 
not  allowed  to  be  swallowed.  It  cannot  be  due  to  loss  of 
hunger,  for  this  is  experimentally  shown  to  affect  all  the 
tentacles  at  once.  Allabach  concludes  that  it  is  simply 
a  case  of  local  fatigue  of  the  tentacles.  The  taking  of  food 
by  a  tentacle  involves  the  production  of  a  considerable 
quantity  of  mucus,  the  immediate  supply  of  which  is  prob- 
ably exhausted  after  a  few  reactions,  and  a  short  period  of 
rest  is  required  (3).  Parker  (551)  is  still  of  the  opinion  that 
adaptation  is  the  proper  explanation  for  the  phenomenon. 

Another  case  of  the  cessation  of  reaction  to  a  repeated 
stimulus  is  reported  by  Wasmann  of  ants  in  an  artificial 
nest,  which  assumed  the  fighting  attitude  in  response  to 
the  movement  of  a  finger  outside  the  nest,  but  after  two  or 
three  repetitions  of  the  motion  were  no  longer  disturbed 
(762).  Where  animals  as  high  in  the  scale  as  the  ant  and 
spider  are  concerned,  it  is  possible  that  this  process  of 
getting  used  to  a  stimulus  may  involve  rather  a  dulling  of 
emotion  than  a  disappearance  of  sensation.  This  phe- 
nomenon also  is  familiar  in  our  experience,  and  may  be 
called  emotional  adaptation. 

That  adaptation  is  itself  adaptive  hardly  needs  to  be 


256  The  Animal  Mind 

emphasized.  As  Jennings  suggests,  if  the  sea-anemone 
that  contracts  at  the  first  ray  of  light  were  to  remain  con- 
tracted in  steady  illumination,  it  would  lose  all  chance  of 
getting  food  under  the  new  conditions  (374).  The  negative 
reactions  ordinarily  involve  interruption  of  the  food-taking 
process,  and  it  is  important  that  they  should  not  be 
continued  in  response  to  stimulation  that  is  relatively 
permanent.  Hargitt  thinks  that  the  loss  of  reaction  to  re- 
peated shadows  which  he  observed  in  marine  worms  may 
be  an  adaptation  to  the  varying  illumination  caused  by 
ripples  at  the  surface  of  the  water  (285). 

A  very  important  psychological  question  concerns  the 
permanence  of  the  effects  of  adaptation.  Sensory  adap- 
tation and  lapse  of  attention  to  repeated  or  continuous 
stimuli,  as  these 'phenomena  are  met  in  our  own  experience, 
are  not  considered  phenomena  of  learning  at  all.  The 
former  is  purely  temporary  in  its  effects :  the  person  who 
has  become  so  used  to  an  odor  that  he  cannot  smell  it  shows 
no  effects  of  this  experience  half  an  hour  later.  The  effect 
of  familiarity  on  emotion  and  on  attention  is  more  lasting : 
one's  loss  of  attention  to  a  clock  ticking  in  one's  room  may 
persist  despite  more  or  less  prolonged  absences  from  the 
room,  although  a  sufficiently  long  absence,  during  which 
one  encountered  no  ticking  clocks,  would  cause  the  sound 
to  be  noticed  again.  The  loss  of  emotional  response  to  a 
familiar  stimulus  may  persist  for  some  time.  Emotional 
adaptation  and  lapse  of  attention  to  continued  stimuli 
may  fairly  be  termed  learning  in  proportion  as  their  effects 
are  more  than  temporary. 

In  many  cases,  the  effects  of  adaptation  on  animal  re- 
actions last  over  a  considerable  interval  between  the  stimuli. 
This  seems  to  be  increasingly  the  case,  the  higher  the  animal. 
Thus  Hydra,  which  is  only  a  ccelenterate,  if  it  is  allowed  to 


Modification  by  Experience  257 

reach  full  expansion  after  having  contracted  at  a  touch,  will 
respond  to  the  second  touch  just  as  it  did  to  the  first ;  the 
stimuli,  to  exert  any  influence  on  later  reactions,  must  come 
in  quick  succession.  On  the  other  hand,  in  the  responses  of 
mollusks  to  shadows,  the  experiences  of  one  day  appear  to 
extend  their  effects  to  the  following  day  (520,  588,  590). 
Here  we  are  dealing  with  a  new  type  of  modification  by 
experience,  though  one  which  develops  directly  out  of  sen- 
sory adaptation ;  namely,  the  relatively  permanent  dropping 
off  of  useless  movements. 


§  73.  Modifications  Due  to  Relatively  Permanent  Effects 

of  Stimuli 

In  true  learning,  the  conscious  experience  and  the  be- 
havior of  an  animal  suffer  changes  so  lasting,  relatively 
speaking,  that  they  cannot  be  set  down  as  due  merely  to 
adaptation  of  the  sense  organ,  muscular  fatigue,  hunger, 
satiety,  or  any  other  variable  physiological  state  of  the 
organism.  On  the  other  hand,  as  we  saw  in  Chapter  II, 
the  modifications  must  occur  rapidly  enough  so  that  there 
is  not  time  for  actual  changes  in  the  animal's  muscular 
structure  to  be  produced.  In  animals  which  possess  nerv- 
ous systems,  true  learning  is  probably  always  the  result 
of  alterations  in  the  connections  between  the  elements  of 
that  system,  such  that  the  nervous  process  is  able  to  pass 
easily  in  a  direction  where  it  originally  encountered  high 
resistances. 

The  fundamental  law  of  all  learning  is  the  Law  of  Repeti- 
tion, whereby  when  a  nervous  process  traverses  a  certain 
pathway  in  the  nervous  system,  it  leaves  the  resistances  in 
that  pathway  less  than  it  found  them.  This  is  the  law 
in  accordance  with  which,  when  we  wish  to  learn  anything, 


258  The  Animal  Mind 

we  repeat  it  over  and  over,  relying  on  the  certainty  that 
each  repetition  will  make  the  next  one  easier.  With  this 
law  in  mind  as  an  essential  postulate,  we  shall  survey  the 
types  of  true  learning  found  in  the  lower  animals  under  the 
following  four  heads :  (i)  learning  involving  the  dropping 
out  of  movements;  (2)  learning  involving  the  formation 
of  series  of  movements ;  (3)  the  recognition  of  landmarks ; 
(4)  learning  involving  the  anticipation  of  movements. 

§  74.  Learning  Involving  the  Dropping  Out  of  Movements 

Among  all  the  movements  which  an  animal  is  capable  of 
making,  there  are  some  which  are  closely  connected  with 
the  great  needs  of  its  existence,  and  others  whose  connec- 
tion with  such  needs  is  only  indirect  and  casual.  The  gen- 
eral process  of  adjustment  to  environment  which  has  made 
the  animal  what  he  is,  has  so  ordered  matters  that  the 
vitally  important  movements  are  in  a  state  of  especial 
readiness  to  be  performed.  The  nervous  resistances  along 
the  pathways  leading  to  the  muscles  used  in  these  move- 
ments are  congenitally  low.  Such  responses  are  what 
Sherrington  (68 i,  p.  229)  has  called  "prepotent  reflexes." 

Now  if  we  survey  all  the  cases  in  which  an  animal  learns 
by  experience,  we  are  obliged  to  conclude  that  on  some 
principle  of  economy  of  energy,  isolated  movements  which 
do  not  bring  any  consequences  of  importance  to  the  organism 
tend  to  be  dropped,  and  their  places  taken  by  a  state  of  rest. 
This  seems  to  be  the  law  according  to  which  we  ourselves 
cease  to  pay  any  attention  to  our  familiar  surroundings. 
We  cease  to  notice  the  ticking  of  a  clock,  although  no  adap- 
tation takes  place  in  the  ear  itself ;  we  sleep  undisturbed 
by  the  noise  of  the  trolley  cars  which  is  distracting  to 
our  friends  from  the  country.  The  spider  experimented 


Modification  by  Experience  259 

on  by  the  Peckhams  reacted  each  day  to  the  sound  of 
a  tuning  fork  by  dropping  from  its  web  until  the 
sound  had  been  repeated  some  half  dozen  times,  but 
after  the  fifteenth  day  it  would  not  drop  at  all  (570). 
Pier  on  (588,  590)  found  that  snails,  while  at  first  respond- 
ing to  shadows  by  withdrawing  the  tentacles,  on  succes- 
sive days  stopped  reacting  after  fewer  and  fewer  trials; 
and  believed  he  could  trace  a  parallel  between  the  laws  of 
this  learning  and  those  of  human  memory.  There  is  no 
question  in  such  cases  of  the  reaction's  being  dropped  off 
in  favor  of  some  other  reaction.  It  is  dropped  off,  as  it 
were,  by  its  own  weight;  simply  because  it  is  useless. 
This  same  principle  seems  to  enter  as  a  cooperating  factor 
in  cases  where  animals  acquire  a  discrimination  between 
stimuli.  The  apparent  ability  of  sea-anemones  to  dis- 
tinguish between  real  food  and  filter  paper  soaked  in  food- 
juice  (see  page  254)  is,  as  we  have  seen,  ascribed  by  some 
to  sensory  adaptation,  but  the  experiments  of  Fleure  and 
Walton  (228),  if  their  results  are  accepted,  would  indicate 
that  true  learning  is  involved.  They  tested  Actinia  with 
a  scrap  of  filter  paper  once  every  twenty-four  hours,  plac- 
ing it  on  the  same  tentacles,  which  usually  carried  it  to 
the  mouth,  where  it  was  swallowed  and  later  ejected. 
After  from  two  to  five  days  the  mouth  would  no  longer 
swallow  the  fragment,  and  in  two  more  days  the  tentacles 
refused  to  take  hold  of  it.  Other  tentacles  could  be  "de- 
ceived" at  least  once  or  twice  after  this,  but  very  soon 
manifested  the  inhibition.  All  traces  of  the  learning  were 
lost  after  from  six  to  ten  days  interval.  Another  anemone, 
Tealia,  learned  more  quickly  than  Actinia.  Again,  Her- 
rick  (297)  found  that  catfish,  when  the  barbels  were  touched 
with  a  bit  of  meat,  immediately  seized  it.  If  a  piece  of 
cotton  wool  were  used  instead  of  the  meat,  they  made  the 


260  The  Animal  Mind 

same  reaction,  but  after  this  experience  had  been  repeated 
a  certain  number  of  times  they  ceased  to  respond  to  the 
cotton,  although  they  still  took  meat  eagerly.  The  point 
which  especially  concerns  us  is  this :  "I  rarely,"  says  Her- 
rick,  "after  the  first  trials,  got  a  prompt  gustatory  reflex 
with  the  cotton."  The  learning  persisted  for  a  day  or 
two.  The  axolotl  learned  in  a  similar  way  to  discriminate 
between  pieces  of  meat  and  pieces  of  wood  (276).  Hermit 
crabs,  which  when  young  try  to  take  up  their  abode  in  all 
sorts  of  unsuitable  objects,  glass  balls,  for  instance,  later 
in  life  make  no  such  efforts  (194). 

Whether  or  not  a  movement  which  brings  no  favorable 
results  will  be  dropped  off  and  a  state  of  no  movement  will 
take  its  place  depends  on  how  strongly  prepotent  the  move- 
ment is ;  upon  the  strength,  that  is,  of  the  innate  tendency  to 
make  it.  In  the  experiments  by  Professor  Bentley  and  the 
writer  on  color  discrimination  in  the  creek  chub,  our  first 
method  failed  because  it  required  the  dropping  off,  as  use- 
less, of  a  strongly  prepotent  reaction,  and  the  substitution 
of  no  response  at  all.  Red  forceps  and  green  forceps,  each 
containing  food,  were  plunged  one  at  a  time  into  the  water ; 
the  fish  was  allowed  to  get  the  food  from  the  red  forceps, 
but  the  green  ones  were  withdrawn  before  it  had  a  chance 
to  bite.  The  time  which  the  fish  took  to  rise  and  snap 
at  the  forceps  was  measured  by  a  stop-watch,  and  in  the 
course  of  131  experiments  the  fish  had  not  learned  to  rise 
to  the  green  any  less  promptly  than  to  the  red.  In  other 
words,  no  tendency  to  drop  off  the  useless  movement  of 
rising  to  the  green  was  detected,  although  later  experiments 
showed  that  the  fish  could  distinguish  between  the  two 
forceps.  The  movement  of  rising  to  and  biting  any  small 
object  in  the  water  was  so  vitally  important  to  the  fish 
that  it  could  not  be  dropped  off  (757).  On  the  other 


Modification  by  Experience  261 

hand,  Thorndike  (708)  successfully  carried  out  this  kind  of 
training  with  Cebus  monkeys  :  both  of  his  subjects  learned 
to  come  down  to  the  bottom  of  the  cage  for  food  when  the 
experimenter  took  the  food  in  his  right  hand,  and  to  stay 
up  when  he  took  it  in  his  left  hand,  the  food  being  withheld 
if  the  monkeys  came  for  it  in  the  second  case.  Cole  (134) 
trained  raccoons  to  climb  up  on  a  box  for  food  when  one  of 
two  differently  colored  cards  was  shown,  and  to  stay  down 
when  the  other  one  appeared,  by  not  feeding  the  raccoons  if 
they  climbed  up  for  the  wrong  card. 

The  dropping  off  of  movements  takes  place  with  more 
speed  and  certainty  if  they  are  made  to  give  place,  not 
simply  to  a  state  of  no  movement  at  all,  but  to  a  movement  of 
greater  prepotency  than  their  own.  Especially  effective  in 
thus  causing  the  elimination  of  a  movement  is  the  negative 
reaction  of  withdrawal  from  injury.  Thus  if  a  movement 
A  results  in  actual  harm  to  the  organism,  the  harmful  stimu- 
lus thus  produced  brings  about  the  negative  response ;  and 
the  negative  reaction  is  as  a  rule  prepotent  over  all  others. 
The  next  time  the  movement  A  is  initiated,  the  negative 
reaction  is  also  initiated,  and  being  prepotent,  it  is  able 
to  check  effectively  the  performance  of  movement  A. 
Thus  we  have  the  dropping  of  of  harmful  movements,  a 
process  which  is  in  evidence  whenever  punishment  is  used 
in  studying  the  learning  power  of  animals.  It  also  ap- 
pears when  a  successful  negative  reaction  permanently 
takes  the  place  of  unsuccessful  ones.  We  saw  in  the  first 
part  of  this  chapter  that  when  an  animal  is  repeatedly  sub- 
jected to  a  strong  and  harmful  stimulus,  it  goes  through 
a  series  of  reactions,  all  directed  to  getting  rid  of  the  stimu- 
lus, until  one  is  finally  successful.  Now  if  this  process  is 
shortened  in  successive  trials,  so  that  the  successful  nega- 
tive reaction  comes  to  be  made  at  once  and  the  unsuccess- 


262  The  Animal  Mind 

ful  ones  dropped  off,  we  have  a  case  where  the  dropping 
off  is  not  simply  of  useless  but  of  harmful  movements  (since 
the  unsuccessful  ones  all  result  in  a  repetition  of  the  harm- 
ful stimulus) ;  the  final  state  is  not  one  of  no  movement, 
but  of  victory  for  the  successful  negative  response.  A 
very  interesting  illustration  of  this  type  of  learning  was 
obtained  from  Paramecium  by  Stevenson  Smith  (688) 
and  by  Day  and  Bentley  (178).  The  method  used  by 
these  experimenters  was  fundamentally  the  same.  A 
glass  tube  was  drawn  out  until  it  was  so  fine  that  not  more 
than  one  Paramecium  could  get  through  it.  This  tube 
was  filled  with  water  up  to  a  certain  point,  and  a  single 
Paramecium,  carefully  isolated  for  identification  through- 
out the  experiment,  was  allowed  to  swim  up  the  tube  until 
the  surface  film  was  reached.  The  animal  behaved  to- 
wards the  film  as  to  any  mechanical  stimulus,  darting 
backward,  rolling  over  towards  the  side  away  from  the 
mouth  and  swimming  forward  again.  Since  the  tube  was 
so  narrow,  this  method,  which  ordinarily  succeeds  in  avoid- 
ing obstacles,  brought  the  animal  against  the  surface  film 
again.  After  repeatedly  going  through  the  same  perform- 
ance, the  Paramecium  varied  its  response  and  succeeded  in 
turning  completely  around  in  the  tube  by  bending  its 
body  double.  On  being  put  again  into  the  same  predica- 
ment, it  gradually  diminished  the  number  of  trials  of  the 
unsuccessful  negative  response,  and  arrived  at  the  point 
where  it  almost  immediately  doubled  over  on  striking  the 
surface  film.  These  observations  established  the  existence 
of  a  relatively  high  type  of  learning  in  the  simplest  group 
of  animals. 

In  this  case  the  movements  that  are  dropped  off  are 
themselves  negative  reactions.  In  other  cases  they  may 
be  feeding  reactions  or  other  responses  whose  vital  impor- 


Modification  by  Experience  263 

tance,  though  great,  cannot  compete  against  that  of  the 
negative  response  called  forth  by  their  injurious  effect  in 
the  special  case.  Learning  by  punishment  is  in  most  cases 
especially  rapid.  Its  effect  may  be  to  inhibit  altogether, 
for  some  time,  a  certain  instinct.  For  example,  the  ex- 
perience of  receiving  an  electric  shock  when  they  seized  a 
certain  kind  of  food  prevented  frogs  from  feeding  at  all 
for  several  days  (657).  Rats  which  were  being  trained  to 
discriminate  between  a  lighter  and  a  darker  passage  with 
the  use  of  an  electric  shock  acquired  a  distaste  for  the  ap- 
paratus as  a  whole  (328).  Mobius  in  1873  (497)  made 
some  experiments  with  a  pike,  afterwards  repeated  by 
Triplett  (720)  with  perch,  which  illustrate  the  same  phenom- 
enon. The  fish  was  kept  in  one  half  of  an  aquarium,  sepa- 
rated by  a  glass  screen  from  the  other  half,  in  which  min- 
nows were  swimming  about.  The  pike  naturally  dashed 
at  them,  and  whenever  it  did  so  bumped  its  nose  on  the 
glass  partition.  After  a  considerable  period  of  this  sort  of 
experience,  the  glass  screen  was  removed,  and  the  min- 
nows were  allowed  to  swim  freely  around  the  pike,  when  it 
was  found  that  the  latter's  instinct  to  seize  them  had  been 
wholly  suppressed  by  the  harmful  consequences  of  such 
action.  Here,  again,  the  chances  that  a  movement  will 
be  suppressed  in  favor  of  the  negative  response  depends 
on  how  great  the  degree  of  its  prepotency  is.  It  was  a 
rash  conclusion  on  Bethe's  (49)  part  to  deny  the  learning 
ability  of  the  crab  because,  although  every  time  it  went 
into  the  darkest  corner  of  its  aquarium  it  was  seized  by  a 
cephalopod  lurking  there,  it  did  not  in  six  such  experiences 
learn  to  inhibit  its  innate  tendency  to  avoid  light :  further 
training  would  probably  have  been  successful.  Yerkes 
(822)  trained  an  earthworm,  by  giving  it  an  electric  shock 
when  it  followed  its  innate  inclination  for  turning  towards 


264  The  Animal  Mind 

a  darkened  region,  to  turn  away  and  towards  the  light. 
The  cockroach,  as  is  well  known,  prefers  darkness  to  light : 
Szymanski  (701),  however,  succeeded,  by  giving  it  an 
electric  shock  when  it  ran  into  the  dark  part  of  a  box,  in 
educating  it  to  turn  back  as  soon  as  it  reached  the  edge 
of  the  darkened  region,  without  waiting  for  the  shock, 
and  Turner  (729)  obtained  similar  results. 

When  an  instinct  is  thus  completely  suppressed  by 
punishment,  the  conscious  accompaniment  of  this  modifi- 
cation in  behavior  is  probably  simply  a  change  in  the  af- 
fective tone  of  the  situation.  Instead  of  being  pleasant, 
it  becomes  unpleasant.  In  a  human  being,  memory  ideas 
might  accompany  the  process  :  a  human  pike,  for  instance, 
might  at  the  sight  of  a  minnow  recall  clearly  the  bump  on 
the  nose  and  his  consequent  humiliation.  But  we  can 
explain  the  pike's  behavior  just  as  well  if,  in  accordance 
with  Lloyd  Morgan's  canon,  we  assume  merely  that  the 
sight  of  a  minnow  has  become  unpleasant  to  him :  he  has 
lost  his  taste  for  minnows. 

Punishment  has  been  the  means  in  many  cases  of  train- 
ing animals  to  manifest  their  ability  to  discriminate  be- 
tween stimuli.  The  desired  end  is  of  course  to  attach  the 
negative  reaction  to  those  features  in  which  the  "wrong" 
stimulus  differs  from  the  "right"  stimulus.  For  instance, 
an  animal  is  being  taught  to  choose  a  light  rather  than  a 
dark  passage,  the  two  openings  being  side  by  side :  when  he 
enters  a  dark  passage  he  gets  an  electric  shock.  It  will  be 
natural  for  him  at  first  to  attach  the  withdrawing  reaction 
consequent  on  the  electric  shock  to  the  sight  of  the  whole 
apparatus.  Whether  he  will  shrink  back  from  it  or  rush 
indiscriminately  into  either  of  the  passages  depends  on 
the  relative  prepotency  of  his  impulse  to  enter  the  pas- 
sages and  his  impulse  to  withdraw  from  injury :  in  either 


Modification  by  Experience  265 

case  he  makes  no  discrimination.  The  discrimination  oc- 
curs when  the  withdrawing  reaction  attaches  itself  to  the 
feature  which  distinguishes  the  dark  passage  from  the  rest 
of  the  apparatus,  namely,  its  darkness.  It  is  probable 
that  in  many  cases  the  animal  does  not  deliberately  com- 
pare the  light  with  the  dark  passage,  but  merely  learns  to 
distinguish  the  passage  to  be  avoided  from  the  rest  of  the 
situation  at  large.  We  should  expect  this  to  be  the  case 
where  punishment  is  the  only  method  of  training  used : 
the  case  would  not  be  one  of  "white  preferred  to  black," 
but  of  "anything  rather  than  black." 

The  strength  of  the  punishment  applied  is  of  course  an 
influential  factor  in  the  learning.  Obviously  it  depends 
not  merely  on  the  strength  of  the  punishing  stimulus,  but 
on  the  sensitiveness  of  the  punished  animal.  Yerkes 
was  the  first  experimenter  to  employ  the  electric  shock  as 
a  means  of  training  animals.  He  used  it  on  the  frog  (805), 
which  he  was  trying  to  educate  to  make  a  turning  to  the 
left  rather  than  to  the  right :  the  frog  showed  a  discouraging 
tendency  to  sit  motionless  for  long  periods  of  time,  and  so 
Yerkes  placed  electric  wires  on  the  floor,  to  induce  by  a 
mild  shock  greater  activity.  In  his  work  on  the  dancing 
mouse  (820),  he  substituted  the  giving  of  electric  punish- 
ments in  the  case  of  wrong  choices,  for  the  older  method  of 
rewarding  an  animal's  right  choices,  and  one  of  the  advan- 
tages claimed  for  this  method  was  that  it  seems  to  allow  an 
exact  measurement  of  the  strength  of  the  stimulus,  whereas 
a  reward,  such  as  food,  varies  in  strength  with  the  animal's 
physiological  condition.  But  the  effect  of  an  electric  shock 
too  varies  with  the  temporary  physiological  state  of  the 
animal,  and  with  its  general  individual  sensibility.  Yerkes 
(827)  carried  out  some  interesting  experiments  on  the  re- 
lation of  the  strength  of  the  punishment  to  the  difficulty 


266  The  Animal  Mind 

of  the  discrimination  required  of  the  animal.  A  super- 
ficial consideration  of  the  situation  might  assume  that  if 
one  wants  to  teach  an  animal  a  difficult  discrimination,  such 
as  that  between  two  slightly  different  shades  of  gray,  one 
ought  to  supply  a  stronger  punishment  stimulus  than 
would  be  necessary  to  teach  it  an  easy  discrimination,  such 
as  that  between  black  and  white.  The  results  with  the 
dancing  mouse  showed  on  the  contrary  that  weaker  pun- 
ishments were  more  effective  in  the  learning  of  hard  dis- 
criminations;  stronger  punishments  in  that  of  easy  dis- 
criminations. The  same  rule  was  found  by  Dodson  to 
hold  for  cats  (186  a) ;  the  hardest  discriminations  were 
acquired  by  kittens  in  82.5  trials  with  a  moderate  stimulus, 
but  107.5  trials  were  required  on  the  average  with  a  strong 
stimulus.  Indications  of  a  similar  relation  were  found  by 
Cole  (136)  in  the  learning  of  chicks. 

The  negative  reaction  is  not  the  only  one  which  may 
show  sufficient  prepotency  to  cause  the  dropping  off  of 
other  responses.  The  feeding  reaction,  or  any  other  in- 
nate response,  may  serve :  thus  reward  as  well  as  punish- 
ment is  a  method  of  training.  The  taming  of  an  animal 
by  kind  treatment  illustrates  both  the  simple  dropping  off 
of  useless  movements,  the  getting  used  to  a  situation,  and 
the  substitution  of  movements  more  valuable  to  the  ani- 
mal; the  tamed  creature  on  the  one  hand  learns  to  rest 
quietly  in  the  presence  of  its  tamer,  instead  of  displaying 
alarm,  and  on  the  other  hand  to  come  for  food  or  follow  for 
companionship.  A  very  pretty  illustration  of  the  over- 
coming of  an  innate  response  to  light  by  the  response  to 
feeding  was  obtained  by  Wodsedalek  (795)  on  immature 
mayflies.  These  insects  have  an  innate  tendency  to  avoid 
light  and  to  remain  under  stones  in  the  water.  By  regu- 
larly feeding  them  on  the  upper  surface  of  a  stone  the 


Modification  by  Experience  267 

experimenter  was  able  wholly  to  overcome  this  reaction, 
especially  with  one  gifted  individual.  After  two  months 
of  training,  "all  that  was  necessary  to  bring  the  specimen 
up  when  it  had  disappeared  from  sight  was  to  slightly  jar 
the  dish  or  the  table  on  which  the  dish  was  located,  and 
the  insect  would  quickly  come  up  to  the  upper  side  of  the 
rock  and  make  for  its  feeding  place.".  Here  again,  the 
conscious  aspect  of  the  learning  is  probably  a  reversal  of 
the  emotional  tone  of  the  situation :  originally  unpleasant, 
it  has  become  pleasant.  Where  the  method  of  reward  is 
used  to  train  animals  in  discriminating  stimuli,  the  influ- 
ence of  the  reward  is  combined  with  that  of  the  tendency 
to  drop  off  useless  movements.  Cole's  raccoons  learned 
not  only  to  climb  up  when  the  food  signal  was  given,  but 
to  stay  down  when  the  no-food  signal  appeared.  The 
rabbits  studied  by  Miss  Abbott  and  the  writer  (756)  were 
taught  to  push  at  a  door  carrying  a  piece  of  red  paper,  and 
to  refrain  from  pushing  at  a  door  carrying  gray  paper. 
The  original  stimulus  for  the  pushing  was  the  odor  of 
food  which  was  in  the  compartments  behind  both  doors. 
The  "gray"  door  was  always  bolted  on  the  inside,  so  that 
pushing  against  it  was  in  vain;  the  "red"  door  opened 
freely  so  that  the  rabbits  could  get  at  the  food.  The  actual 
securing  of  the  food  acted,  along  with  the  smell  of  it,  to 
suppress  all  useless  hesitations  on  the  part  of  the  animals 
and  to  make  them  more  inclined  to  push  the  doors  at  once ; 
the  gray  stimulus  acquired  a  tendency  to  lose  its  motor 
effect  because  the  movements  to  which  it  gave  rise  were 
useless. 

So-called  "puzzle-box"  experiments  also  depend  for 
their  training  effect  upon  the  combined  tendencies  to  the 
survival,  through  their  prepotency,  of  movements  result- 
ing in  the  satisfaction  of  an  instinct,  and  to  the  dropping 


268 


The  Animal  Mind 


off  of  useless  movements.  The  method  has  been  tried 
with  birds,  rats,  squirrels,  cats,  dogs,  raccoons,  porcupines, 
and  monkeys.  Thorndike,  its  originator,  made  some 
experiments  of  this  type  on  chicks  confined  in  pens 
from  which  they  could  be  released  by  pecking  at  a 
string  or  some  such  object  (704).  Porter  tested  English 
sparrows  with  boxes  containing  food,  which  could  be  en- 


FIG.  12. —  Puzzle  box  used  in  Porter's  work  on  birds;  AB,  one  method  of  attach- 
ing string  to  latch ;  C,  a  second  method.  In  the  first,  the  loop  at  B  had  to  be 
pulled ;  in  the  second,  the  string  had  to  be  pushed  in. 

tered  by  pulling  a  string  fastened  to  a  latch,  or  by  pushing 
the  string  into  the  wire  netting  with  which  one  side  of  the 
box  was  covered  (Fig.  16).  The  sparrows  learned  very 
quickly;  one  of  them  by  the  tenth  test  had  left  out  all 
unnecessary  movements  (610).  In  later  experiments  a 
cowbird  and  a  pigeon  also  learned  to  open  a  similar  box. 
Before  beginning  the  test  the  birds  were  accustomed  to 
being  fed  in  the  box  with  the  door  open.  Their  first  suc- 
cess in  opening  the  door  lay  in  accidentally  clawing  or 
pecking  at  the  proper  point,  and  in  later  trials  the  action 


Modification  by  Experience  269 

was  simplified ;  thus  the  birds  learned  not  to  attack  other 
parts  of  the  box,  to  use  the  bill  instead  of  the  claws,  and 
to  stand  on  the  floor  beside  the  box  instead  of  hopping  upon 
it  (611).  In  Rouse's  test  of  the  pigeon  by  the  puzzle-box 
method,  it  showed  less  aptitude  than  that  displayed  by 
the  English  sparrow  (647). 

Small  (684)  tested  his  white  rats  with  two  boxes  contain- 
ing food.  One  could  be  entered  by  digging  away  the  saw- 
dust which  was  banked  around  the  lower  end  of  the  box, 
if  the  digging  was  done  in  a  particular  place ;  the  other, 
by  tearing  off  strips  of  paper  which  held  shut  a  spring  door. 
The  result  of  the  earlier  series  of  experiments  with  the  first- 
mentioned  box  was  that  after  an  hour  and  a  half  on  the 
first  day  one  rat  happened  to  dig  in  the  right  place  and 
entered.  The  second  day  this  rat  took  only  eight  min- 
utes, and  the  thirteenth  day  only  thirty  seconds,  to  enter. 
With  the  second  box  there  was  always  a  tendency  to  be- 
gin by  digging,  and  even  in  the  thirteenth  experiment, 
where  the  rat  got  in  by  biting  off  the  papers  in  fifteen  seconds, 
she  began  by  two  strokes  of  digging.  In  a  later  test  with 
this  box  the  rat  chanced  to  be  extremely  hungry,  and  dug 
violently  for  several  seconds,  displaying  a  blunting  of 
the  discriminative  powers  by  hunger,  analogous  to  that 
which  we  have  found  in  very  low  animals.  The  rats 
were  later  trained  to  discriminate  between  the  two  boxes, 
being  sometimes  presented  with  one  and  sometimes  with 
the  other. 

In  Thorndike's  work  on  cats  and  dogs,  the  investigator 
placed  the  animals  themselves  in  the  boxes,  and  food  on 
the  outside,  so  that  the  problem  was  not  how  to  get  in  but 
how  to  get  out.  The  getting  out  could  be  accomplished 
in  various  ways,  such  as  pulling  a  wire  loop,  clawing  a 
button  around,  pulling  a  string  at  the  top  of  the  box,  poking 


270 


The  Animal  Mind 


a  paw  out  and  clawing  a  string  outside,  raising  a  thumb 
latch  and  pushing  against  the  door,  and  so  on  (Fig.  13). 
The  animals,  on  being  first  put  into  the  box,  made  all  sorts 
of  movements  in  their  struggles  to  get  out ;  the  right  move- 
ment was  hit  upon  by  accident.  Only  very  gradually,  as 
the  experiment  was  repeated  again  and  again,  were  the 
useless  movements  omitted,  until  finally  the  right  one  was 


FIG.  13.  —  Puzzle  box  used  in  Thorndike's  experiments  on  cats. 

performed  at  once  (704).  Wesley  Mills  criticised  these 
pioneer  experiments  of  Thorndike's  on  the  ground  that  the 
animals  were  under  such  unnatural  conditions  and  in  such 
an  extreme  state  of  hunger  that  they  profited  by  experi- 
ence more  slowly  than  might  otherwise  have  been  the  case 
(492) ;  and  this  may  have  been  to  a  certain  extent  true. 
In  testing  monkeys  with  puzzle  boxes  Thorndike  placed 
the  food  on  the  inside  and  the  monkeys  on  the  outside.  He 
found  a  marked  difference  between  the  speed  of  their 
learning  and  that  shown  by  the  cats  and  dogs.  "Whereas 
the  latter  were  practically  unanimous,  save  in  the  cases  of 
the  very  easiest  performances,  in  showing  a  process  of 


Modification  by  Experience 


271 


gradual  learning  by  a  gradual  elimination  of  unsuccessful 
movements  and  a  gradual  reenforcement  of  the  successful 
one,  these  are  unanimous,  save  in  the  very  hardest,  in 
showing  a  process  of  sudden  acquisition  by  a  rapid,  often 
apparently  instantaneous  abandonment  of  the  unsuccess- 
ful movements  and  selection  of  the  appropriate  one,  which 
rivals  in  suddenness  the  selec- 
tions made  by  human  beings 
in  similar  performances" 
(708).  Kinnaman  further 
complicated  the  box  tests 
with  his  Macacus  monkeys 
by  constructing  "  combina- 
tion" fastenings,  which  re- 
quired the  performance  of  a 
set  of  actions  in  a  certain 
order,  and  found  that  these 
were  mastered  by  the  animals 
(401)  (Fig.  14). 

Cole's  (134)  work  on  the 
raccoon  indicates  that  in  speed  of  learning  this  animal 
stands  "almost  midway  between  the  monkey  and  the 
cat,"  while  "in  the  complexity  of  the  associations  it 
is  able  to  form  it  stands  nearer  the  monkey."  The  rac- 
coons, like  the  monkeys,  learned  combination  locks,  al- 
though they  did  not  learn  to  perform  the  various  move- 
ments involved  in  a  definite  order.  They  showed  an  in- 
teresting tendency  to  skip  at  once  to  the  movement  that 
immediately  preceded  the  opening  of  the  door.  The 
porcupine  also  proved  gifted  with  the  ability  to  learn  com- 
bination locks  (651),  while  the  squirrel's  puzzle-box  ex- 
ploits were  limited  to  boxes  which  could  be  entered  by  the 
simple  process  of  digging  in  sawdust  (832).  The  learning 


FIG.  14.  —  Combination  fastening  used 
in  Kinnaman's  work  on  monkeys. 
The  figures  indicate  the  order  in 
which  the  parts  of  the  combination 
had  to  be  dealt  with. 


2*72 


The  Animal  Mind 


of  combination  locks  probably  involves  the  formation  of 
systems  of  movement,  as  well  as  the  dropping  off  of  useless 
movements;  the  process  of  system  formation  will  be  dis- 
cussed in  a  later  section. 

The  building  up  of  systems  of  movements  is  an  important 
part  of  the  learning  process  in  another  method  of  studying 

the  intelligence  of 
animals,  namely,  the 
labyrinth  or  maze 
method.  In  the 
typical  form  of  this 
method,  food  is 
placed  at  the  end 
of  a  pathway  in- 
volving a  number 
of  turnings,  in  which 
it  is  possible  to  make 
errors  of  two  sorts : 

FIG.  15 — The  Hampton  Court  maze.  (#)    taking    a    longer 

instead  of  a  shorter  route,  (b)  entrance  into  cul-de-sacs. 
The  animal  has  to  learn  to  run  to  the  end  of  the  path 
and  secure  the  food  in  the  shortest  possible  time,  or  by 
the  most  direct  "route.  His  progress  in  learning  may  be 
measured  either  by  the  total  time  he  consumes  in  run- 
ning the  path  in  each  trial,  or  by  the  number  of  errors  he 
makes,  or  by  the  total  distance  he  runs.  The  method  in 
its  developed  form  was  first  used  by  Small  (685)  in  experi- 
ments on  white  rats,  and  js  especially  adapted  to  an  animal 
so  active  as  the  rat.  Small  used  a  very  complicated  maze, 
a  facsimile  on  a  small  scale  of  the  one  to  be  found  in  the 
grounds  of  Hampton  Court  Palace  (Fig.  15).  Such  mazes, 
with  high  box  walls,  were  a  frequent  feature  of  old  gar- 
dens. Much  simpler  mazes  have  been  used  with  other 


Modification  by  Experience  273 

animals.  Where  a  maze  consists  of  only  two  passages,  re- 
quiring the  animal  to  learn  merely  a  single  turning,  the 
method  maybe  practically  merely  a  discrimination  method  : 
thus  Yerkes's  (822)  training  of  the  earthworm  made  use  of  a 
maze  with  two  passages  only  to  choose  between,  a  light  and 
a  dark  one.  In  a  pure  maze  experiment,  however,  there 
is  no  way  of  distinguishing  between  the  passages  except 
by  experiencing  the  consequences  of  following  them.  Thus 
the  crayfish  was  tested  by  the  use  of  a  maze  with  a  single 
choice  of  paths.  One  end  of  the  box  communicated  with 
the  aquarium ;  about  halfway  down  the  length  of  the  box 
a  partition  put  in  longitudinally  divided  it  into  two  pas- 
sages, one  of  which  was  closed  at  the  end  by  a  glass  plate. 
In  sixty  trials  the  animals,  which  had  originally  chosen  the 
correct  passage  50  per  cent,  of  the  time,  came  to  choose  it 
90  per  cent,  of  the  time.  A  second  series,  with  a  single 
animal  upon  which  more  tests  a  day  were  made,  resulted 
in  the  formation  of  a  perfect  habit  in  two  hundred  and  fifty 
experiments.  The  glass  plate  was  then  shifted  to  the  other 
passage,  and  the  crayfish  was  naturally  completely  baffled 
for  a  time,  but  succeeded  in  learning  the  new  habit  (829). 
The  crab  Carcinus  granulatus  made  progress  in  learning  to 
traverse  a  labyrinth  with  two  points  where  a  choice  of  path 
had  to  be  made,  but  did  not  wholly  master  it  in  fifty 
trials  (804).  For  both  the  crab  and  the  crayfish,  the 
experience  of  getting  back  into  the  water  was  the  influence 
relied  upon  to  eliminate  the  useless  movements ;  the  slow 
learning  of  these  animals  indicates  that  the  method  was  not 
well  adapted  to  them.  Ants  showed  some  ability  to  learn 
a  maze  with  several  turning  points,  following  the  proper 
course  even  when  their  smell  trail  was  obliterated  (219). 
Fish  have  proved  able  to  learn  very  simple  labyrinths,  but 
have  not  been  tested  in  complicated  ones  (705,  121). 


274 


The  Animal  Mind 


With  the  green  frog,  a  maze  allowing  two  choices  was 
used,  and  was  learned  in  one  hundred  trials  (805). 

With  the  turtle,  a  labyrinth  distinctly  more  complex 
was  used.  It  involved  four  blind  passages,  and  led  to 
the  turtle's  comfortable,  darkened  nest.  During  the 
first  four  trips  the  time  was  reduced  from  thirty-five 

minutes     to     three 


minutes  and  thirty 
seconds;  in  the 
fourth  trip  the  an- 
imal took  two  wrong 
turns.  The  time  of 
the  fiftieth  trip  was 
thirty-five  seconds. 
In  a  second  laby- 
rinth (Fig.  1 6),  two 
inclined  planes  were 
introduced,  up  and 
down  which  the 
turtles  had  to  crawl. 
This  labyrinth  took 


FIG.  16.  —  Labyrinth  used  by  Yerkes  with  turtles. 
A,  starting  point;  F,  blind  alley;  3,  4,  6,  in- 
clined planes. 


them  longer  to  traverse,  and  the  time  curve  shows  greater 
irregularity,  rising,  for  instance,  to  seven  minutes  on  the 
forty-fifth  trial,  after  having  been  as  low  as  two  minutes 
and  forty-five  seconds  at  the  thirty-fifth.  The  process  of 
shortening  the  path  was  observed  very  prettily  in  connec- 
tion with  the  inclined  planes.  The  turtles  had  to  turn 
about  as  soon  as  they  had  reached  the  bottom  of  the  de- 
scending plane.  They  soon  began  to  make  the  turn  be- 
fore they  got  to  the  bottom,  and  finally  to  throw  them- 
selves over  the  edge  as  soon  as  they  reached  the  top  (80 1). 
Some  of  Thorndike's  (704)  early  experiments  on  chicks 
involved  a  very  simple  form  of  the  labyrinth  method,  in 


Modification  by  Experience  275 

that  the  chicks,  which  were  confined  in  small  pens,  could 
escape  by  running  to  a  particular  spot,  or  up  an  inclined 
plane.  Porter  (610)  found  that  the  English  sparrow  quickly 
learned  the  Hampton  Court  maze,  and  that  the  vesper  spar- 
row and  cowbird  learned  a  simpler  form  in  twenty  or  thirty 
trials  (611).  Pigeons  tested  by  Rouse  acquired  the  ability 
to  traverse  four  different  labyrinths,  and  it  was  noted  that 
their  experience  with  the  earlier  ones  seemed  to  help  them 
in  the  later  ones  (647).  Hunter's  (349)  experiments  on  the 
pigeon  with  the  use  of  the  maze  will  be  mentioned  later. 

White  rats  observed  by  Small  learned  the  Hampton  Court 
maze,  in  nine  experiments  made  at  intervals  of  two  days, 
so  well  that  they  committed  only  two  errors  in  the  ninth 
test,  but  the  significance  of  this  time  is  obscured  by  the 
fact  that  the  rats  were  allowed  to  run  freely  about  the 
labyrinth  every  night  (684). 

In  Yerkes's  (820)  study  of  the  Japanese  dancing  mouse, 
the  reactions  to  irregular  and  to  regular  labyrinths  were 
compared,  and  it  was  found  that  a  maze  of  the  latter  type, 
that  is,  one  where  left  and  right  turns  alternated,  was 
more  quickly  learned  and  more  perfectly  mastered  than  an 
irregular  one.  Kinnaman  (401)  taught  two  Macacus 
monkeys  the  Hampton  Court  maze. 

The  feature  of  maze  learning  which  interests  us  at  this 
point  is  the  dropping  off  of  useless  movements.  This 
probably  occurs  partly  through  the  general  tendency  of 
useless  movements  to  be  omitted,  and  partly  through  the 
tendency  of  the  successful  movements  to  survive.  It  has 
been  argued  that  if  the  shortening  of  the  maze  running  is 
due  to  the  fortunate  consequences  of  the  successful  move- 
ments, then  the  errors  which  should  be  earliest  dropped 
off  are  those  at  the  latter  part  of  the  course,  which  come 
nearest  in  point  of  time  to  the  final  success,  usually  the 


276  The  Animal  Mind 

obtaining  of  food.  Rubber t  (346)  does  not  find  that  this 
is  actually  the  case,  but  Vincent  (750)  does,  and  on  the 
whole  the  evidence  points  to  the  conclusion  that  the  errors 
nearest  the  final  success  are  the  first  ones  eliminated. 
Such  exceptions  to  this  tendency  as  appear  may  well  be 
due  to  two  causes :  first,  the  natural  tendency  of  useless 
movements  to  drop  away  even  when  a  successful  movement 
is  not  pushing  them  out,  and  secondly,  the  equally  natural 
but  wholly  opposed  tendency  of  movements  to  organize 
themselves  into  systems,  a  tendency  which  will  be  con- 
sidered in  the  next  section.  Watson  (771)  lays  especial 
stress  on  the  fact  that  the  successful  movements  in  puzzle- 
box  and  maze  experiments  have  the  advantage  of  fre- 
quency of  performance.  The  successful  movements  are 
always  performed,  in  every  maze  experiment,  simply  be- 
cause the  experiment  continues  until  they  are  performed ; 
there  is  no  such  necessity  that  any  particular  unsuccessful 
movement  should  be  performed  in  every  experiment.  Thus 
the  successful  movements,  Watson  thinks,  owe  their  survival 
to  the  law  of  repetition.  It  is  quite  probable  that  their  in- 
evitable performance  once  in  each  running  of  the  maze  may 
be  a  factor  aiding  their  survival,  although  quite  conceivably, 
as  Thorndike  *  has  suggested,  many  unsuccessful  movements 
may  actually  be  oftener  performed,  owing  to  the  fact  that 
they  may  be  repeatedly  tried  in  the  same  experiment. 
But  Watson  endeavors  to  reduce  all  learning  through  the 
dropping  off  of  movements  to  the  influence  of  the  frequency 
with  which  the  successful  movements  occur ;  and  this  can 
only  be  done  by  ignoring  such  cases  of  learning  as  those 
where  the  frog  ceased  in  one  or  two  trials  to  snap  at  food 
when  the  snapping  led  to  harmful  consequences,  or  where 
the  spider  learned  not  to  disturb  itself  at  the  sound  of  a 

1  Jour.  Animal  Behav.,  vol.  5  (1915),  p.  465. 


Modification  by  Experience  277 

tuning  fork.  Frequency  cannot  be  a  factor  of  importance 
here.  Such  cases  show  that  there  do  exist  in  animals  ten- 
dencies (a)  to  abandon  movements  which  have  no  conse- 
quences of  importance  to  the  organism,  and  (b)  to  eliminate 
even  movements  that  are  important  in  favor  of  movements 
that  have  greater  importance.  It  is  not  clear  why  Watson 
is  unwilling  to  admit  that  the  "sensory  consequences," 
the  vital  importance  of  the  results  of  a  movement,  are 
factors  in  determining  its  survival.  He  seems  to  think 
that  sensory  consequences  must  be  stated  in  terms  of 
mental  processes,  and  therefore  must  not  be  mentioned  by 
a  consistent  "behaviorist."  We,  not  being  behaviorists 
but  psychologists,  are  quite  willing  to  talk  about  the  pleas- 
antness and  unpleasantness  accompanying  the  benefit  and 
harm  of  reactions,  but  if  we  were  behaviorists,  we  should 
certainly  not  feel  obliged  to  deny  that  animals  can  be  ben- 
efited or  harmed  by  their  own  actions,  because  we  feared 
benefit  and  harm  might  suggest  pleasantness  and  unpleas- 
antness to  the  minds  of  our  readers. 

Before  we  pass  on  to  another  aspect  of  learning,  which 
is  quite  as  important  in  the  phenomena  of  maze  running 
as  the  dropping  off  of  movements,  there  are  a  few  points 
to  be  noted  with  regard  to  the  relation  between  punishment 
and  reward,  harm  and  benefit,  as  influences  in  learning. 
Punishment  appears  to  produce  more  rapid  learning  than 
reward,  unless  it  is  so  severe  that  it  attaches  itself  to  the 
whole  learning  situation.  Punishment  and  reward  com- 
bined give,  probably,  better  results  than  either  alone  (328). 
Further,  a  movement  that  results  in  harm,  and  is  therefore 
supplanted  by  the  negative  response  of  withdrawal,  is  more 
completely  eliminated  than  one  which  is  merely  useless 
and  is  supplanted  simply  by  a  state  of  rest.  Evidence  of 
this  was  obtained  by  Bogardus  and  Henke  (59)  in  experi- 


278  The  Animal  Mind 

ments  where,  after  rats  had  learned  a  maze,  the  path  was 
altered,  certain  passages  being  closed  and  others  opened. 
The  rats  found  it  decidedly  harder  to  learn  to  enter  former 
cul-de-sacs  than  to  take  those  turnings  which  they  had 
formerly  omitted  merely  because  they  were  a  longer  way 
around  than  the  true  path.  The  positively  unpleasant 
experience  of  running  into  a  cul-de-sac  had  more  com- 
pletely eliminated  the  movements  that  led  to  it  than  did 
the  merely  useless  running  of  a  longer  passage.  Finally, 
it  is  clear  that  we  cannot  draw  a  hard  and  fast  line  between 
a  useless  reaction  and  a  harmful  one.  We  have  seen  that 
a  severe  punishment  like  an  electric  shock  may  delay  learn- 
ing because  it  attaches  itself  to  the  learning  situation  as 
a  whole.  And  in  the  writer's  experiments  "with  rabbits 
(756),  a  young  rabbit  of  very  nervous  temperament  was 
rendered  unfit  for  further  experimentation  simply  by  hap- 
pening to  push  repeatedly  at  the  wrong  or  closed  door  of 
a  box.  He  had  been  working  well  up  to  that  time,  but  from 
that  time  on  he  ran  away  whenever  he  was  confronted  with 
the  experiment  box.  It  would  appear  that  emotional 
factors  in  the  animal  may  render  movements  positively 
harmful  which  would  ordinarily  be  merely  useless. 

§  75.   The  Formation  of  Systems  of  Successive  Movements 

We  have  now  to  consider  another  type  of  learning,  dia- 
metrically opposed  to  that  of  the  dropping  off  of  movements. 
In  this  type,  the  movements  which  an  animal  makes  suc- 
cessively become  organized  into  a  series.  No  movement  of 
the  series  is  dropped  out  as  a  result  of  the  learning,  but  the 
oftener  the  series  is  run  through,  the  more  rapidly  it  is 
performed.  It  is  evident,  if  we  consider  our  own  learning 
processes,  that  many  of  them  are  of  this  type.  When  we 


Modification  by  Experience  279 

say  the  alphabet  or  the  multiplication  table,  the  learning 
process  has  not  involved  the  dropping  out  of  any  of  the 
movements.  It  would  profit  us  little  to  pass  immediately 
from  A  to  Z,  dropping  out  the  intervening  movements,  or  to 
skip  at  once  from  the  first  to  the  last  stanza  of  a  poem.  We 
find  in  such  cases  that  repeated  performance  of  the  series 
of  movements  results  in  two  changes.  First,  the  move- 
ments follow  each  other  more  rapidly :  this  can  be  ex- 
plained by  the  law  of  repetition,  according  to  which  the 
oftener  the  nervous  process  traverses  a  certain  pathway, 
the  less  resistance  it  encounters.  Secondly,  the  movements 
of  the  series  no  longer  need  outside  stimuli,  but  apparently 
each  movement  supplies  the  stimulus  for  the  next.  When 
one  is  playing  a  piece  of  music  for  the  first  or  second  time, 
each  movement  has  to  have  the  stimulus  of  the  notes  on 
the  page;  when  a  piece  has  been  long  practised,  each 
movement  sets  up  the  next  one  '  automatically.'  This 
really  means  that  as  one  movement  is  performed,  the  sen- 
sory processes  occasioned  by  the  contraction  of  the  muscles 
involved  excite  the  motor  pathway  for  the  next  movement. 
The  stimulus  for  one  movement  is  the  kinsesthetic  excita- 
tions received  from  the  preceding  movement.  The  truth 
of  this  is  evidenced  by  the  fact  that  if  we  break  down  in 
playing  a  certain  passage,  we  can  recover  ourselves  by 
going  back  a  little,  so  as  to  get  the  proper  kinaesthetic 
stimuli. 

This  type  of  learning  obviously  functions  in  learning  a 
maze  path.  Here  we  have  to  deal  not  with  the  acquiring 
of  a  single  'successful'  movement  and  the  dropping  off  of 
all  others,  but  with  the  establishment  of  a  whole  series  of 
successful  movements  which  must  be  performed  in  a  cer- 
tain order.  Much  experimentation  has  been  performed  to 
study  the  sensory  cues  involved  in  maze  running.  The 


280  The  Animal  Mind 

question  resolves  itself  into  three :  (i)  What  sensory  stimuli 
does  an  animal  rely  upon  after  it  has  thoroughly  learned 
the  maze  path?  (2)  What  sensory  stimuli  does  an  animal 
naturally  rely  upon  in  learning  the  maze  path  ?  (3)  What 
sensory  stimuli  can  an  animal  do  without,  if  necessary,  in 
learning  a  maze  path? 

(1)  All  the  evidence  indicates  that  when  a  rat  has  thor- 
oughly learned  the  maze,  its  movements  have  become  or- 
ganized into  a  system  such  that  the  sole  requisite  stimulus 
for  the  performance  of  one  movement  is  the  kingesthetic 
excitation  resulting  from  the  preceding  movement.     The 
best  proof  of  this  is  furnished  by  experiments  where  the 
rats  which  had  learned  the  maze  from  its  beginning  were 
started  at  points  further  along;    they  could  not  pick  up 
the  true  path  with  any  speed  until,  in  running  back  and 
forth,  they  chanced  to  make  two  or  three  correct  turnings. 
This  set  off  the  remainder  of  the  maze-running  process  pre- 
cisely as  playing  over  the  preceding  passage  would  enable 
a  pianist  to  proceed  beyond  the  point  of   a   breakdown. 
Similarly,  when  the  maze  was  shortened  by  removing  a 
section  from  its  middle,  the  rats  ran  against  the  ends  of  the 
shortened  passages ;   when  it  was  lengthened  by  elongating 
certain  passages,  the  rats  tried  to  make  the  turns  at  the  old 
points  (118). 

(2)  and  (3)    It  is  difficult  to  make  sure  just  what  sen- 
sory stimuli  function  in  enabling  an  animal  to  learn  the 
maze.     The  only  methods  that  suggest  themselves  are  (a) 
that   of   depriving   an   animal   of   the   use  of   one   sense 
and  seeing  whether  he  can  learn  the  path,  thus  answering 
question    (3)    above;     or    (b)    that    of     supplying    him 
with  cues  especially  appealing  to  a  certain   sense,   and 
noting  whether  his  learning  is  accelerated.     But  neither 
of  these  methods  reproduces  the  condition  of  normal  maze 


Modification  by  Experience  281 

learning.  Watson  (767)  showed  that  rats  deprived  of 
sight,  hearing,  smell,  and  touch  (the  vibrissae  or  long 
whiskers  being  removed  and  the  paws  made  anaesthetic), 
could  learn  the  maze.  Yerkes  (820)  demonstrated  that 
the  Japanese  dancing  mouse  does  not  necessarily  depend 
on  sight,  smell,  or  touch  for  guidance.  On  the  other  hand, 
Vincent's  (747-749)  experiments  show  that  a  maze  in  which 
the  true  path  was  painted  black  and  the  wrong  paths  white, 
or  vice  versa,  was  learned  more  quickly  than  an  ordinary 
maze ;  that  one  in  which  the  true  path  was  smeared  with 
beef  extract  and  cream  cheese  alternately  (the  two  odors 
being  used  to  prevent  olfactory  adaptation)  gave  a  greater 
total  accuracy ;  and  that  in  a  maze  without  sides,  that  is, 
an  elevated  pathway,  the  rats  were  much  disturbed  by 
the  loss  of  the  accustomed  contact  with  the  walls.  Dif- 
ferent animals  are  undoubtedly  unlike  in  the  use  they  make 
of  sensory  cues.  The  frog  studied  by  Yerkes  (805)  in  a 
very  simple  labyrinth  showed  a  disturbance  in  its  habit 
when  red  and  white  cards  placed  on  either  side  of  the  pas- 
sage were  interchanged.  The  pigeon  (647),  when  required 
to  go  through  a  labyrinth  in  darkness,  was  obliged  to  relearn 
it.  On  the  other  hand,  Small  found  that  altering  the  direc- 
tion of  the  light  had  little  effect  on  the  performances  of 
his  white  rats.  He  also  placed  wooden  pegs  painted  red, 
at  each  division  of  the  paths,  in  the  middle  of  the  correct 
path,  and  caused  the  maze  thus  arranged  to  be  learned  by 
untrained  rats.  They  did  not  learn  it  any  faster  because 
of  the  presence  of  these  visual  hints,  nor,  when  it  had  been 
learned,  were  they  at  all  discomposed  by  the  removal  of 
the  pegs  (684).  Allen's  (4)  guinea  pigs  did  not  alter  their 
behavior  when  the  position  of  colored  cards  in  the  maze 
was  changed.  Rouse  (647)  found  that  the  pigeon  could 
make  use  of  auditory  stimuli  as  cues.  He  arranged  to  have 


282  The  Animal  Mind 

an  electric  bell  rung  whenever  the  birds  entered  a  wrong 
alley,  and  a  wooden  bell  sounded  when  they  emerged  and 
took  the  right  course.  After  they  had  learned  the  path 
under  these  conditions  the  two  kinds  of  sound  stimuli 
were  interchanged,  and  the  result  was  a  certain  amount 
of  confusion  on  the  part  of  the  birds.  On  the  whole,  in 
the  case  of  active  animals  whose  vision  is  not  highly  de- 
veloped, such  as  the  rat,  the  principal  factor  in  learning  a 
maze  appears  to  be  the  actual  running  of  it.  As  the  paths 
are  traversed  at  random,  the  useless  movements  tend  to 
be  dropped  off,  and  the  successful  ones  not  merely  to  sur- 
vive, but  to  become  organized  into  a  system  such  that  each 
movement  itself  provides  the  stimulus  for  the  succeeding 
one.  Vincent  (747)  found  that  while  visual  cues  aided 
the  learning  of  the  maze,  the  final  running  was  not  so  rapid 
as  if  the  habit  had  been  formed  wholly  under  kinaesthetic 
guidance. 

Some  curious  results  have  been  observed  when  the  maze 
is  rotated  through  angles  of  90,  180,  or  270  degrees.  Since 
this  has  no  effect  on  any  of  the  paths,  but  only  on  the 
relation  of  the  entire  maze  to  its  environment,  it  ought  not 
to  disturb  animals  which  are  depending  entirely  on  their 
own  movements  for  their  cues,  yet  apparently  it  does  in 
some  cases  disturb  them  (767).  Possibly  the  preliminary 
swings  which  the  animal  gets  in  being  picked  up  and  intro- 
duced to  the  entrance  of  the  maze  are  the  disturbing  factor. 
Hunter  (349)  found  that  some  of  his  pigeons  were  disturbed 
when  the  maze  was  rotated,  while  others  were  not,  and 
concludes  that  the  latter  were  guided  by  cues  within  the 
maze,  the  former  by  cues  from  without. 

Maze  experiments  are  not  the  only  observations  on  ani- 
mals which  reveal  the  existence  of  successive  movement 
systems,  or  "  kinaesthetic  memory."  If  Pieron  (579)  is 


Modification  by  Experience  283 

right,  some  species  of  ants  are  aided  in  their  return  to  the 
nest  by  repeating  all  the  turnings  they  took  on  the  out- 
ward journey.  Watson  reports  the  following  observation 
on  the  terns  of  the  Tortugas.  On  an  occasion  after  he 
had  trained  one  of  these  birds  to  use  a  nest  raised  a  hundred 
centimeters  above  the  ground,  he  moved  the  nest  a  hundred 
centimeters  to  the  eastward ;  the  bird,  returning,  hovered 
"in  space,  attempting  to  adjust  to  the  nest  in  the  air  at  its 
former  position  and  height"  (769,  page  226).  Rock- 
well (639)  relates  that  a  ground  squirrel  had  made  inside 
a  cabin,  a  nest  which  it  was  accustomed  to  reach  by  climb- 
ing up  the  leg  of  a  cot  that  stood  in  one  corner  of  the 
cabin.  When  the  cot  was  removed,  the  squirrel,  entering 
the  cabin,  ran  to  the  place  formerly  occupied  by  the  cot, 
and  went  through  the  motions  of  trying  to  climb  the  non- 
existent leg. 

It  must  further  be  noted  that  useless  movements  not  in- 
frequently get  organized  into  movement  systems,  and  thus 
their  elimination  is  delayed.  Animals  in  running  a  maze 
form  habits  of  going  wrong  which  greatly  interfere  with 
the  reduction  of  their  time  records.  In  the  case  of  some 
salamanders  which  the  writer  vainly  tried  to  teach  a  fairly 
complicated  maze,  each  individual  acquired  quite  an  elab- 
orate habit  of  making  wrong  turnings,  and  remained  true 
to  it  for  some  time.  Whenever  the  situation  does  not  in- 
volve strongly  prepotent  movements,  whenever,  that  is, 
the  "  motive"  is  weak,  the  natural  tendency  of  movements  to 
organize  into  systems  may  take  the  place  of  the  tendency 
to  drop  off  the  unnecessary  ones.  Such  an  influence  as 
this  is  very  likely  one  reason  why  errors  in  the  maze  are 
not  eliminated  in  the  exact  order  of  their  distance  from  the 
final,  beneficial,  and  pleasurable  goal. 

The  conscious  accompaniment  of  the  formation  of  sue- 


284  The  Animal  Mind 

cessive  movement  systems  is,  in  our  own  experience  and 
probably  in  that  of  an  animal,  the  diminishing  of  atten- 
tion to  outside  stimuli,  and  the  disappearance  of  such  emo- 
tional states  as  uncertainty  and  the  unpleasantness  of 
errors ;  a  feeling  of  confidence  and  security  replacing  them. 
There  is  an  alleged  case  of  learning  on  the  part  of  certain 
marine  animals  which,  if  it  exists,  probably  belongs  under 
the  head  of  the  formation  of  successive  systems.  This  is 
the  acquisition  of  rhythmic  reactions,  to  stimuli  which 
occur  at  equal  time  intervals,  and  the  persistence  of  such 
rhythms  when  the  stimuli  have  ceased  to  act.  Marine 
snails,  sea-anemones,  annelid  worms,  and  hermit  crabs 
show  changes  in  the  direction  of  their  responses  to  light 
and  to  gravity  which  correspond  with  the  state  of  the  tides : 
the  sea-anemone,  for  instance,  opens  at  high  tide  and  shuts 
at  low  tide.  Now  certain  French  observers,  Bohn  (80,  90), 
Pieron  (584),  and  Drzewina  (192)  report  that  when  the 
animals  are  removed  to  the  aquarium,  they  continue  to 
show  fluctuations  in  their  light  and  gravity  reactions  at 
the  times  of  high  and  low  tide,  although  of  course  the  actual 
stimuli  which  the  tide  gives  them,  for  instance  the  mechani- 
cal jarring  of  the  waves  entering  their  pool  as  the  tide  rises, 
are  now  wholly  lacking.  No  American  observer  has  been 
able  to  show  such  a  continuance  of  the  tidal  rhythm  in 
animals  removed  from  direct  tidal  action  (256,  293,  509  a, 
551).  The  phenomenon  suggests  analogies  from  our  own 
experience;  for  instance,  there  is  the  case  of  "habit  hun- 
ger." We  feel  hungry  at  the  time  of  day  when  we  are  ac- 
customed to  be  fed ;  if  we  do  not  get  food  at  this  time,  in 
half  an  hour  or  so  the  hunger  sensations  disappear,  and 
we  can  go  quite  comfortably  without  food  for  some  time 
longer.  The  hunger  sensations  are  due  to  movements  of 
the  stomach;  now  these  movements  were  originally  in- 


Modification  by  Experience  285 

duced  by  the  presence  of  food  in  the  stomach  at  a  certain 
time.  They  have  apparently  become  a  part  of  a  system 
of  internal,  organic  movements,  so  that  when  these  internal 
processes  have  continued  for  a  length  of  time  equal  to  that 
which  usually  elapses  between  meals,  they  produce  the 
stomach  contractions,  in  the  absence  of  the  original  stimulus, 
the  food.  Thus  the  case  seems  to  be  like  that  of  the  run- 
ning of  the  maze  by  a  thoroughly  practised  animal ;  each 
act  is  the  stimulus  for  the  next,  and  outside  stimuli  are 
unnecessary. 


CHAPTER  XI 

THE  MODIFICATION  OF  CONSCIOUS  PROCESSES  BY  INDI- 
VIDUAL EXPERIENCE  (Continued} 

§  76.   The  Recognition  of  Landmarks 

A  TYPE  of  learning  which  stands  by  itself  is  that  involved 
in  the  homing  of  certain  animals.  As  we  have  seen,  the 
evidence  is  conclusive  that  solitary  wasps  guide  themselves 
back  to  the  nests  they  have  made  by  " recognizing"  certain 
visual  peculiarities  of  the  surroundings.  They  are  con- 
fused if  the  appearance  of  the  nest  or  its  vicinity  is  al- 
tered. On  first  leaving  the  nest  in  search  of  the  prey  with 
which  to  stock  it,  as  food  for  the  larva,  they  make  an  elabo- 
rate flight  with  many  turnings  in  and  out  about  the  im- 
mediate neighborhood,  which  has  been  appropriately 
termed  a  locality  survey.  Now  when  the  wasp  has  found 
and  secured  the  caterpillar  or  spider  which  she  seeks,  she 
retraces  her  flight  apparently  with  the  guidance  of  the  visual 
landmarks  she  noted  on  the  outward  journey.  No  one,  it 
is  true,  has  yet  actually  determined  the  homeward  flight 
of  the  wasp  in  its  relation  to  landmarks,  but  the  probabili- 
ties are  that  such  is  her  method  of  procedure.  The  peculi- 
arity of  such  learning  is  that  it  does  not  depend  on  repeti- 
tion. The  wasp  makes  but  one  nest  in  a  given  situation, 
and  in  the  case  of  certain  species  at  least  she  makes  but  one 
flight  in  search  of  food  and  but  one  homing  flight.  She 
then  makes  a  new  nest  in  a  new  locality,  impresses  new 
landmarks  upon  her  memory,  and  is  guided  in  her  next 

286 


Modification  by  Experience  287 

homing  flight  by  the  new  and  not  the  old  landmarks.  The 
learning  is  essentially  rapid  and  temporary.  Where,  as 
for  instance  with  the  honey  bee,  the  nest  remains  per- 
manently fixed  in  one  locality,  guidance  by  visual  land- 
marks does  not  differ  from  the  ordinary  types  of  learning 
where  the  process  is  gradual,  where  useless  movements 
are  eliminated  and  useful  movements  organized  into  sys- 
tems. We  are  still  in  possession  of  too  few  detailed  obser- 
vations on  the  homing  flights  of  the  wasp  to  draw  positive 
conclusions  as  to  the  nature  of  the  learning  process  here. 

§  77.    The  Memory  Idea 

It  is  sufficiently  clear  that  animals  possess  the  power  of 
learning,  in  the  sense  of  a  power  of  reacting  differently  to 
a  present  stimulus  because  of  their  past  experience  with  it. 
Probably  not  a  single  animal  form  is  so  low  that  it  lacks 
this  power.  But  there  is  another  type  of  learning,  of  which 
human  beings  make  much  use,  whose  existence  in  animals 
we  have  yet  to  investigate ;  namely,  the  ability  to  recall 
a  mental  image  of  an  absent  stimulus,  a  memory  idea.  A 
dog  shows  clearly  that  he  remembers  his  master,  in  the 
sense  of  modifying  his  behavior  in  his  master's  presence 
because  of  his  previous  experience.  Can  we  be  sure  that 
he  has  remembered  him  in  his  absence ;  that  he  has  had  a 
memory  image  of  his  master? 

Most  people,  following  the  tendency  to  humanize  ani- 
mals and  ignoring  Lloyd  Morgan's  canon,  interpret  as  evi- 
dence of  memory  ideas  certain  features  of  animal  behavior 
which  are  susceptible  of  much  simpler  interpretations. 
Dogs  and  cats,  for  instance,  are  supposed  to  dream  because 
they  snarl  and  twitch  their  muscles  in  sleep ;  but  as  Thorn- 
dike  (704)  has  pointed  out,  such  movements  may  be  purely 


288  The  Animal  Mind 

reflex  and  unaccompanied  by  any  consciousness  whatever. 
A  dog  shows  depression  during  his  master's  absence,  but 
his  state  of  mind  may  be  merely  vague  discomfort  at  the 
lack  of  an  accustomed  set  of  stimuli,  not  a  clear  idea  of 
what  he  wants ;  as  when  we  feel  that  we  have  forgotten 
something  or  that  something  in  our  environment  has  been 
altered. 

We  shall  first  consider  certain  pieces  of  evidence  which 
indicate  that  in  many  of  the  lower  animals  the  existence  of 
memory  ideas  is  highly  doubtful.  Later,  we  shall  note 
certain  testimony  in  favor  of  their  existence  in  the  minds 
of  some  animals,  although  probably  with  a  very  restricted 
function. 

One  argument  from  which  we  may  conclude  that  animals 
do  not  make  use  of  memory  ideas  where  human  beings  would, 
is  derived  from  the  gradual  character  of  the  dropping  off 
of  useless  movements  in  experiments  of  the  puzzle-box 
type.  A  human  being  who  had  once  hit  by  accident  on 
the  right  way  to  open  a  lock  could  hardly  fail,  on  being 
confronted  with  the  lock  a  second  time,  to  recall  an  idea  of 
the  successful  movement,  and  to  perform  it  at  once,  with- 
out wasting  time  and  effort  on  unnecessary  movements; 
but  a  dog  or  a  cat  makes  almost  as  many  random  clawings 
and  pawings  the  second  time  as  the  first,  and  only  gradu- 
ally omits  the  irrelevant  motions. 

In  the  next  place,  animals  very  generally  show  a  lack  of 
ability  to  imitate  other  animals  when  the  "imitatee"  is  not 
actually  present  before  them ;  they  cannot  imitate  by  re- 
membering another  animal's  movements.  Imitation  may 
be,  as  various  authors  have  pointed  out,  of  at  least  two  dif- 
ferent types.  The  first  may  be  called  instinctive  imitation, 
and  is  widespread  throughout  the  animal  kingdom.  It 
occurs  when  the  sight  or  sound  of  one  animal's  performing 


Modification  by  Experience  289 

a  certain  act  operates  as  a  direct  stimulus,  apparently 
through  an  inborn  nervous  connection,  to  the  perform- 
ance of  a  similar  act  by  another  animal.  "If,"  says  Lloyd 
Morgan,  "one  of  a  group  of  chicks  learns  by  casual  experi- 
ence to  drink  from  a  tin  of  water,  others  will  run  up  and 
peck  at  the  water  and  will  themselves  drink.  A  hen  teaches 
her  little  ones  to  pick  up  grain  or  other  food  by  pecking  on 
the  ground  and  dropping  suitable  materials  before  them, 
the  chicks  seeming  to  imitate  her  actions.  .  .  .  Instinc- 
tive actions,  such  as  scratching  the  ground,  are  performed 
earlier  if  imitation  be  not  excluded"  (507,  pp.  166-167). 
Imitation  in  this  sense  is  hardly  so  much  a  method  of  learn- 
ing by  experience  as  a  method  of  supplying  experience. 
An  animal  may  perform  an  act  the  first  time  because, 
through  inherited  nervous  connections,  the  sight  of  another 
animal's  performing  it  acts  as  a  stimulus.  But  it  will  con- 
tinue to  perform  the  act,  in  the  absence  of  any  copy  to 
imitate,  only  if  the  act  is  itself  an  instinctive  one,  like 
drinking  in  birds,  or  becomes  permanent  by  reason  of  its 
consequences,  just  as  would  be  the  case  if  its  first  perform- 
ance had  been  accidental  rather  than  imitative.  As  a 
matter  of  fact,  instinctive  imitation  seems  usually  to  be 
concerned  with  actions  themselves  instinctive. 

Inferential  imitation,  or  what  Morgan  calls  reflective  imita- 
tion, is  a  different  affair.  It  is  the  case  where  an  animal, 
watching  another  one  go  through  an  action  and  observing 
the  consequences,  is  led  to  perform  a  similar  act  from  a 
desire  to  bring  about  the  same  result.  Such  behavior 
naturally  suggests  that  it  is  accompanied  by  some  kind  of 
memory  idea  of  the  action  that  is  imitated.  Now  Thorn- 
dike,  in  his  experiments  on  chicks,  cats,  and  dogs,  found  no 
evidence  of  this  type  of  imitation.  A  cat  put  in  a  puzzle- 
box  did  not  learn  the  way  out  any  sooner  for  watching, 
u 


290  The  Animal  Mind 

even  repeatedly,  the  performances  of  a  cat  that  knew  how 
to  get  out  (704).  With  monkeys,  Thorndike's  most  ex- 
tensive tests  were  made  to  find  whether  the  animal  would 
learn  to  open  a  box  from  seeing  the  experimenter  himself 
do  it,  and  his  results  were  again,  on  the  whole,  negative 
(708).  Small's  white  rats  also  showed  no  ability  to  profit 
by  each  other's  experience  in  this  way.  One  of  each  of  the 
pairs  first  experimented  on  solved  the  problems  presented  ; 
the  other,  instead  of  either  attacking  them  for  itself  or  learn- 
ing by  watching  the  successful  one,  contented  itself  with 
stealing  the  food  secured  by  the  latter  (685).  Imitation, 
according  to  Yerkes,  plays  no  considerable  role  in  the 
learning  processes  of  the  dancing  mouse  (820).  Where 
an  animal  is  not  at  all  helped  to  the  solution  of  a  problem 
by  watching  another  animal  solve  it,  we  are  justified  in 
concluding  that  if  it  can  recall  memory  ideas  at  all,  it  does 
not  make  use  of  them  in  a  situation  where  a  human  being 
would  certainly  do  so. 

The  lack  of  ability  on  an  animal's  part  to  postpone  reacting 
to  a  stimulus  is  another  evidence  of  inability  to  make  use 
of  memory  ideas.  The  very  ingenious  method  by  which 
such  ability  may  be  studied  was  the  device  of  Hunter 
(350).  It  has  been  termed  the  Delayed  Reaction  Method, 
and  its  general  plan  is  as  follows.  A  light  is  shown  for 
a  few  seconds  in  any  one  of  three  directions  from  the 
animal,  which  is  restrained  from  reacting.  After  the  light 
is  turned  off  and  a  certain  time  interval  has  elapsed,  the 
animal  is  released,  and  if  it  goes  in  the  direction  in  which 
the  light  appeared,  it  receives  food.  Now  white  rats  could 
succeed  in  running  in  the  proper  direction  when  the  delay 
between  the  disappearance  of  the  light  and  their  release 
was  not  more  than  ten  seconds,  but  only  in  case  they 
pointed  their  noses  at  the  light  when  it  appeared  and 


Modification  by  Experience  291 

remained  motionless  in  this  position  during  the  interval. 
Clearly  we  get  no  indication  from  such  behavior  that  the 
rat  is  able  to  recall  a  memory  image  of  the  light.  His 
failure  to  run  in  the  right  direction  when  he  did  not  keep 
his  nose  pointed  in  the  right  direction  plainly  suggests  the 
absence  of  such  ideas  as  influences  on  his  behavior. 

Again,  the  nature  of  the  errors  which  animals  occasionally 
make  in  experiments  strongly  suggests  the  absence  of  mem- 
ory ideas.  Thus  the  two  rats  which  learned  the  Hampton 
Court  maze  under  Small's  (685)  tuition  both  continued, 
after  they  had  reached  their  highest  point  of  excellence  in 
running  the  maze,  to  take  the  wrong  turning  at  the  outset. 
Precisely  this  error  would  have  been,  probably,  the  first 
one  eliminated  in  the  learning  of  a  human  being,  who  would 
be  able  to  recall  some  kind  of  memory  idea  of  the  first 
turning  owing  to  its  especial  hold  upon  attention.  Fur- 
ther, the  way  in  which  instinctive  actions  are  often  performed 
by  animals  indicates  that  ideas  are  not  present  as  they 
would  be  to  a  human  being's  consciousness.  Human  beings 
do  some  things  from  instinct,  but  the  doing  of  them  may  be 
accompanied  by  ideas;  a  mother's  care  for  her  child  in- 
volves ideas  of  the  child's  happiness  or  suffering,  and  of  its 
future.  Enteman's  account  of  the  worker  wasp  which, 
lacking  other  food  to  present  to  a  larva,  bit  off  a  portion  of 
one  end  of  the  larva's  body  and  offered  it  to  the  other  end 
to  be  eaten,  suggests  a  peculiar  limitation  of  ideas  in  the 
wasp's  mind,  at  least  while  this  particular  function  was  being 
performed  (206).  The  cow,  which  had  lamented  at  being 
deprived  of  her  calf,  and  on  having  the  stuffed  skin  of  her 
offspring  given  to  her,  licked  it  with  maternal  devotion 
until  the  hay  stuffing  protruded,  when  she  calmly  devoured 
the  hay  (504,  p.  334),  had  perhaps  experienced  some 
dim  ideas  connected  with  her  loss,  but  certainly  her  con- 


292  The  Animal  Mind 

sciousness  was  more  absorbed  by  the  effects  of  present 
stimulation  and  less  occupied  with  ideas  than  a  human 
mother's  would  have  been. 

Thorndike  (704)  was  the  first  to  point  out  how  scanty  is 
the  evidence  in  favor  of  the  possession  of  ideas  by  the 
lower  animals.  In  addition  to  the  fact  that  his  dogs  and 
cats  dropped  off  their  useless  movements  so  slowly,  he  ad- 
duced the  observation  that  while  after  a  time  the  cats  which 
had  been  caused  to  enter  a  puzzle-box  and  let  themselves 
out  before  being  fed  would  of  their  own  accord  go  into  the 
box,  cats  that  had  been  from  the  first  dropped  into  the  box 
at  the  top  never  learned  to  go  in  of  their  own  accord.  He 
argued  that  if  a  cat  had  been  able  to  have  the  idea  of  being 
in  the  box,  as  a  necessary  prelude  to  food,  it  would  have 
been  able  to  pass  from  the  idea  of  being  dropped  in  to  that 
of  going  in  itself.  This  argument,  however,  is  not  fully  con- 
vincing. The  experience  of  being  picked  up  and  dropped 
into  a  box  is  very  different  from  that  of  walking  through 
a  door.  To  the  human  mind,  accustomed  to  more  re- 
fined analysis  of  its  experiences,  one  of  these  would  suggest 
the  other,  but  we  cannot  argue  that  because  such  a  con- 
nection is  not  made  in  the  animal's  mind,  therefore  the 
latter  is  incapable  of  ideas,  any  more  than  we  could  con- 
clude a  total  absence  of  ideas  from  the  consciousness  of  a 
man  to  whom  a  primrose  by  the  river's  brim  does  not 
suggest  thoughts  of  the  moral  government  of  the  universe. 
Moreover,  several  observers  have  reported  precisely  this 
ability  to  get  the  habit  of  jumping  into  a  box  from  being 
dropped  in;  our  rabbits  (756),  which  were  put  into  a  box 
for  safe  keeping  between  experiments,  within  two  days 
acquired  the  trick  of  running  to  the  box  and  scrambling 
into  it,  the  whole  experience  being  a  prelude  to  food. 

The  same  comments,  precisely,  apply  to  Thorndike's 


Modification  by  Experience  293 

observation  that  his  dogs  and  cats  were  not  helped  to  learn 
a  puzzle-box  mechanism  by  being  put  through  the  move- 
ments. The  absence  of  ability  to  pass  from  the  experience 
of  being  put  through  a  movement  to  the  idea  of  performing 
the  movement  is  no  proof  of  incapacity  to  form  ideas; 
moreover  Cole  (134)  found  that  the  raccoon  did  learn  to 
work  a  fastening  by  being  put  through  the  movements. 
Hunter  (351)  made  a  similar  observation  on  the  rat,  and 
the  method  seems  to  meet  with  success  in  the  hands  of 
animal  trainers. 

In  general,  however,  we  must  admit,  the  facts  point 
to  the  conclusion  that  ideas  are  very  rare  in  the  animal  mind. 
We  can  in  some  cases,  however,  present  positive  evidence 
of  their  occurrence.  One  attempt  to  demonstrate  them, 
that  of  Cole  (134),  it  is  true,  seems  hardly  conclusive.  Cole 
trained  raccoons  to  discriminate  between  various  stimuli. 
Cards  were  placed  on  levers  so  that  by  a  touch  they 
could  be  pushed  up  and  down.  The  animals  learned  to 
climb  up  for  food  when  one  of  two  differently  colored 
cards  was  shown,  and  to  stay  down  when  the  other  one 
appeared;  to  distinguish  in  a  similar  way  between  a 
high  and  a  low  tone,  between  a  round  and  a  square 
card,  and  between  a  card  6^  X  6|  inches  and  one  4!  X  4^ 
inches  square.  Of  course  the  action  of  climbing  up  was 
not  itself  purely  instinctive,  but  had  become  associated 
with  the  food  instinct.  The  raccoons  also  hit  upon 
the  trick  of  clawing  up  the  cards  themselves,  and  if 
the  one  that  appeared  was  the  "no-food  "card,  they  would 
either  claw  it  down  again  and  pull  up  the  other,  or  proceed 
at  once  to  pull  up  the  other,  leaving  the  " no-food"  one 
also  up.  Since  the  cards  were  shown  successively,  Cole 
concludes  that  "remembrance  of  the  card  just  shown  was 
required  for  a  successful  response."  "Why,"  he  asks, 


294  The  Animal  Mind 

"should  the  animal  put  the  red  card  down  if  it  did  not  fail 
to  correspond  with  some  image  he  had  in  mind,  and  why 
when  he  put  the  green  up  should  he  leave  it  up  and  go  up 
on  the  high  box  for  food  if  the  green  did  not  correspond 
with  some  image  he  had  in  mind?"  It  seems  to  the  writer 
that  the  supposition  of  an  image  is  unnecessary,  except 
possibly  in  the  experiments  requiring  discrimination  of 
sizes.  It  is  perfectly  possible,  as  we  know  from  our  own 
experience,  to  react  to  one  stimulus  and  not  to  another 
without  going  through  a  comparison  of  the  two,  unless 
the  difference  between  them  is  merely  one  of  degree.  It 
might  have  been  possible  for  a  human  being  to  discriminate 
between  the  larger  and  the  smaller  cards  only  by  calling  up 
a  memory  image  of  the  card  not  shown  and  comparing  it 
with  the  one  before  him ;  it  surely  would  not  have  been 
necessary  for  him  to  use  images  in  the  reactions  to  colors, 
forms,  and  tones.  And  if  a  human  being,  accustomed  to 
much  dependence  on  memory  ideas,  could  get  on  without 
them  here,  surely  a  raccoon  could.  Even  in  judgments  of 
degree,  all  laboratory  psychologists  know  that  human 
beings  have  a  strong  tendency  to  make  absolute  rather 
than  comparative  judgments,  and  use  memory  ideas  but 
little.  Better,  though  still  unsatisfactory,  evidence  of 
the  use  of  images  is  furnished  by  the  following  method: 
"  Three  levers  were  placed  on  the  displayer.  One,  on  being 
raised,  displayed  white,  another  orange,  another  blue. 
The  plan  was  to  display  white,  orange,  and  blue  consecu- 
tively, then  to  display  the  same  blue  three  times.  I  fed 
the  animal  if  he  climbed  upon  the  high  box  on  being  shown 
the  series  white,  orange,  blue,  and  did  not  feed  him  after 
the  series  blue,  blue,  blue."  That  is,  the  stimulus  immedi- 
ately preceding  the  reaction  was  the  same  in  both  cases. 
The  difference  lay  in  the  foregoing  stimuli.  The  series 


Modification  by  Experience  295 

"white,  blue,  red,  food"  and  "red,  red,  red,  no  food"  was 
also  used.  The  raccoons  learned  to  respond  properly, 
"though,"  Cole  continues,  "I  never  completely  inhibited 
the  animals'  tendency  to  start  up  on  seeing  white  or  blue, 
which  were  precursors  of  the  red  which  meant  food.  Thus 
the  animals  all  anticipated  red  on  seeing  its  precursors, 
which  in  itself  seems  good  evidence  of  ideation.  Many 
times,  however,  they  turned  back  after  starting  at  blue  or 
white  and  looked  for  the  red,  then  climbed  up  once  more, 
thus  showing  that  the  red  was  not  a  neglected  element  of 
the  situation,  but  an  expected  color  which  they  generally 
waited  to  see,  but  sometimes  were  too  eager  to  wait  for." 
Certain  details  of  the  raccoons'  behavior  are  significant. 
"Each  one,  on  seeing  the  first  red,  would  drop  down  from 
a  position  with  both  front  paws  on  the  front  board  to  stand 
on  all  fours  in  front  of  it,  and  merely  glance  up  at  the  suc- 
ceeding reds.  As  soon  as  the  white  appeared,  however, 
the  animal  would  lean  up  against  the  front  board,  claw 
down  the  white  and  blue,  but  never  the  final  red.1' 

Now  Cole  thinks  that  the  learning  of  this  trick  by  the 
raccoons  proved  that  "the  animal  retains  an  image  of  the 
cards  which  just  preceded  red."  The  only  alternate  sup- 
position seems  to  him  to  be  that  they  always  reacted  to  the 
number  of  the  card  in  the  series,  which,  if  the  series  were 
irregularly  given,  would  not  have  been  the  same  in  suc- 
cessive trials.  To  suggest  one's  own  interpretation  of 
animal  behavior  that  one  has  not  seen,  in  the  place  of  the 
experimenter's  interpretation,  requires  some  temerity,  but 
to  the  present  writer  the  most  natural  way  of  accounting 
for  the  raccoon's  performances  would  be  the  supposition 
that  in  the  series  white,  blue,  red,  for  instance,  at  the  end 
of  which  they  were  fed,  the  occurrence  of  white  threw  them 
into  a  state  of  expectancy,  of  readiness  to  climb  up  on  the 


296  The  Animal  Mind 

box;  this  was  heightened  by  the  blue,  and  finally  "dis- 
charged" into  action  by  the  red.  During  this  process 
they  may  have  had  an  anticipatory  image  of  the  blue  and 
of  the  red,  although  there  is  no  evidence  that  they  did. 
But  when  the  red  came  they  did  not  stop  to  call  up  memory 
images  of  the  preceding  colors,  and  decline  to  act  until 
they  had  assured  themselves  that  those  were  blue  and 
white  instead  of  red.  Preparedness  to  act  was  probably 
already  secured  by  the  actual  occurrence  of  the  white  card 
at  the  beginning  of  the  series.  In  other  words,  while 
images  may  have  been  present,  they  were  images  with  a 
future,  not  a  past  reference.  A  human  being  reacting  to  a 
series  of  stimuli  in  this  fashion  would  but  rarely,  in  case 
his  attention  had  wandered  during  the  giving  of  the  first 
two  stimuli,  have  to  recall  them  as  memory  images  be- 
fore reaction,  but  he  might  very  likely  have  anticipatory 
images  of  the  stimuli  to  come  while  waiting  for  them. 
These  criticisms,  which  appeared  in  the  first  edition  of 
the  present  work,  were  later  repeated  by  Gregg  and  Mc- 
Pheeters  (268  a),  who  made  experiments  similar  to  Cole's. 
In  favor  of  the  functioning  of  ideas  in  monkeys  and 
raccoons  is  the  fact  that  in  learning  to  open  puzzle-boxes, 
they  drop  off  useless  movements  with  great  speed.  And 
monkeys  have  given  clear  evidence  of  inferential  imitation. 
Kinnaman  (401)  reports  that  in  one  of  his  experiments, 
where  the  box  had  to  be  opened  by  pulling  out  a  plug,  a 
monkey  failed  to  work  the  mechanism  and  gave  up  in 
despair.  Another  monkey  then  came  out  of  the  cage,  the 
first  one  following.  Number  two  went  to  the  box,  seized 
the  end  of  the  plug  with  his  teeth,  and  pulled  it  out.  The 
box  was  set  again,  and  monkey  number  one  rushed  to  it, 
seized  the  plug  as  number  two  had  done,  and  got  the  food. 
She  immediately  repeated  the  act  eight  times.  A  second 


Modification  by  Experience  297 

and  similar  observation  was  made  where  the  mechanism 
was  a  lever.  Haggerty  (281),  as  the  result  of  long  obser- 
vation and  experimenting  on  the  monkeys  in  the  Bronx 
Zoo,  got  some  excellent  instances  of  inferential  imitation, 
of  which  one  may  be  quoted.  The  act  to  be  performed 
was  that  of  climbing  up  the  side  of  the  cage,  thrusting  the 
arm  up  inside  a  wooden  chute,  and  pulling  a  string  inside 
it,  as  a  result  of  which  food  came  tumbling  down.  Mon- 
key number  13  was  allowed  to  watch  monkey  number  4 
go  through  this  process  four  times.  "Number  4  was  now 
removed  and  Number  13  was  released  in  the  cage.  At 
first  he  looked  about  over  the  floor  for  food  and  then 
climbed  the  front  wire,  stopping  on  the  brace  opposite  the 
chute.  He  leaned  over  to  the  chute  and  while  still  stand- 
ing on  the  brace  with  his  feet,  tried  to  thrust  a  hand  into 
the  bottom  of  the  chute.  Failing  in  this,  he  ran  along  the 
brace  . . .  and  back  again  to  opposite  the  chute ;  catching 
the  rung  of  the  chute  in  his  hands  he  drew  himself  over  to 
it ;  finding  himself  above  the  end  of  the  chute  he  tried  to 
let  his  body  down,  first  on  one  side  and  then  on  the  other, 
until  in  the  most  awkward  manner  he  managed  to  get  near 
enough  to  the  end  to  thrust  a  hand  up  the  inside  far  enough 
to  reach  the  string.  At  once  he  pulled  and  the  food  came 
tumbling  down  on  his  chest  and  to  the  floor.  Dropping  to 
the  floor  he  picked  up  the  food  and  ate  it"  (281,  pp. 
360-361).  Such  persistence  of  endeavor  to  carry  out  a 
definite  act  would  certainly  in  a  human  being  be  guided 
by  ideas. 

Again,  in  Hunter's  (350)  work  by  the  Delayed  Reaction 
Method,  the  raccoons  showed  behavior  which  would  seem 
to  indicate  the  presence  of  a  memory  idea.  Although 
they  could  not  go  in  the  right  direction  if  more  than  twenty- 
five  seconds  had  elapsed  since  the  light  was  turned  off, 


298  The  Animal  Mind 

they  succeeded  within  this  interval  whether  they  did  or  did 
not  change  the  position  of  their  bodies.  "Each  of  these 
animals  could  react  successfully  when  the  wrong  orientation 
was  held  at  the  moment  of  release,  and  when,  so  far  as 
the  experimenter  could  detect,  no  part  of  the  animal's 
body  remained  constant  during  the  interval  of  delay" 
(p.  43).  Thus,  after  the  light  was  turned  off,  and  they  had 
moved  about  during  the  period  of  delay,  when  they  were 
released  they  could  move  in  the  direction  where  they  had 
seen  the  light.  The  same  type  of  behavior,  but  extending 
over  much  longer  periods  of  delay,  was  characteristic  of 
children  in  similar  tests,  and  would  seem  to  be  naturally 
accompanied  by  memory  ideas,  although  Hunter  prefers 
to  speak  of  the  re-arousal  of  " intra-organic  cues."  In 
the  present  writer's  opinion,  all  ideas  are  accompanied  by 
"  intra-organic "  or  kinaesthetic  cues.  We  shall  refer  later 
to  this  point.1 

Another  experimental  method  which,  like  the  Delayed 
Reaction  Method,  has  been  devised  to  study  the  possible 
functioning  of  ideas  in  various  animals  is  the  Multiple 
Choice  Method.  Its  beginnings  are  to  be  found  in  the 
work  of  Hamilton  (283).  As  he  used  it,  the  essential 
features  were  as  follows.  The  animal  was  placed  in  a 
compartment  with  four  exit  doors.  All  of  these  doors 
were  locked  except  one,  and  that  one  might  be  any  one  of 
the  four  except  the  door  that  was  open  in  the  previous 
experiment.  The  object  of  the  test  was  to  see  whether  or 
riot  the  animal  approached  comprehension  of  this  prin- 
ciple. The  subjects  were  a  normal  man,  a  defective  man, 

1 A  curious  type  of  delayed  reaction,  which  must  await  further  investi- 
gation, is  reported  by  Mast  (471)  of  the  firefly  Photinus  pyralis.  The 
flash  of  a  female  firefly  causes  the  male  to  move  in  her  direction.  The 
turning  of  the  male  occurs  after  the  female  has  flashed. 


Modification  by  Experience  299 

six  boys  of  varying  ages,  one  defective  boy,  five  monkeys, 
sixteen  dogs,  seven  cats,  and  a  horse.  Only  the  human  sub- 
jects reached  a  stage  of  learning  where  they  showed  by 
their  behavior  that  they  realized  the  impossibility  of  open- 
ing a  door  that  had  been  open  in  the  preceding  trial.  The 
monkeys  always  tried  all  four  doors,  but  did  not  often  push 
repeatedly  at  the  same  door  or  persistently  neglect  a  door ; 
this  lowest  type  of  behavior  was  more  frequent  in  the  horse. 
The  fact  may  be  noted  for  future  reference  that  the  behavior 
of  the  horse  in  this  situation  was  "stupider"  than  that  of 
any  of  the  other  subjects. 

Yerkes  (826)  developed  the  principle  of  this  method 
and  generalized  it  as  follows.  The  animal  is  offered  the 
choice  among  a  number  of  compartments.  The  number 
can  be  varied,  and  their  position  in  space  can  be  varied. 
Thus,  if  there  are  ten  compartments  in  the  apparatus, 
only  three  of  them  may  be  used  in  a  certain  experiment, 
and  these  three  may  be  situated  in  the  middle  or  towards 
either  end,  so  that  no  associations  will  be  formed  with  po- 
sition in  space.  Or  in  another  experiment  five  of  the  com- 
partments, in  any  part  of  the  series,  may  be  used.  The 
compartments  used  in  a  given  experiment  have  their  en- 
trance doors  open.  The  problem  may  be  varied  in  com- 
plexity by  making  the  "right"  compartment,  the  one  whose 
entrance  gives  food,  bear  different  relations  to  the  rest. 
It  may  be  the  first  compartment  on  the  left,  the  first  com- 
partment on  the  right,  the  second  on  the  left,  the  second 
on  the  right,  the  middle  compartment,  and  so  on.  After 
an  animal  has  proved  its  ability  to  learn  a  simple  problem, 
such  as  "first  on  the  right,"  it  may  be  advanced  to  a  more 
complex  one,  such  as  "second  on  the  left."  The  method 
has  been  applied  to  crows  (129),  rats  (113),  pigs  (826), 
monkeys,  and  apes  (824).  The  crow  mastered  the  "first  at 


300  The  Animal  Mind, 

the  right"  and  "first  at  the  left"  problems,  but  failed  in  five 
hundred  trials  to  master  the  "second  at  the  left"  problem. 
The  white  rat  succeeded  with  the  "first  at  the  right,"  but 
failed  with  the  "second  from  the  left"  problem.  The  pig 
distinguished  itself  by  mastering  "first  at  right,"  "second 
from  left,"  "alternately  first  at  left  and  first  at  right," 
failing  only  to  grasp  the  "middle  compartment"  problem. 
The  two  monkeys  tested  by  Yerkes  (824)  showed  im- 
provement in  dealing  with  the  problems  "first  at  left," 
"second  from  right,"  "alternately  first  at  left  and  first  at 
right,"  and  "middle,"  but  appeared  to  owe  many  of  their 
successes  to  their  acquired  preferences  and  aversions  for 
particular  compartments.  The  "alternating"  problem 
proved  to  be  especially  easy.  An  orang-utan,  who  showed 
himself  in  other  tests  the  most  intelligent  of  Yerkes's  sub- 
jects, failed  to  improve  in  solving  the  problems  of  the 
Multiple  Choice  Method.  His  wrong  choices  were  so 
persistent,  and  so  independent  of  the  usual  tendency  to 
drop  off  useless  movements,  that  Yerkes  concluded  him  to 
be  really  acting  on  the  basis  of  wrong  ideas  as  to  the  cor- 
rect solution  of  the  problem.  It  is  clear  that  a  human  be- 
ing who  had  formed  an  incorrect  theory  as  to  the  proper  way 
to  work  out  a  problem  would  take  longer  to  solve  it  than  an 
animal  who  learned  merely  by  the  dropping  off  of  useless 
movements,  provided  that  the  animal  could  solve  it  at  all. 
We  may  now  examine  the  relation  of  the  Multiple  Choice 
Method  to  the  question  of  the  existence  of  memory  ideas 
in  animals.  In  the  first  place,  if  the  "right"  compartment 
always  occupied  the  same  position  in  space,  clearly  an  ani- 
mal might  learn  to  go  to  it  without  the  use  of  memory  ideas. 
Kinaesthetic  memory,  the  formation  of  a  habit  of  turning 
in  a  certain  direction,  would  suffice.  Next,  if  the  correct 
compartment  is  not  always  in  the  same  absolute  position 


Modification  by  Experience  301 

in  space,  but  is  always  the  furthest  to  the  right  or  left  of 
all  the  compartments  used  in  the  experiment,  the  learning 
is  still  easy.  The  animal  has  only  to  combine  the  habit  of 
turning  to  the  right  or  left  with  the  observation  as  to  what 
compartments  have  their  entrance  doors  raised  :  a  compart- 
ment with  closed  doors  offers  no  stimulus.  Thirdly,  even 
the  problem  " second  from  the  left,"  or  right,  might,  it 
would  appear,  be  solved  without  the  use  of  a  memory  idea. 
The  learning  need  involve  only  (a)  the  habit  of  turning  to 
the  left  or  right,  and  (b)  the  habit  of  reacting  negatively  to 
the  open  door  furthest  in  this  direction.  The  natural  re- 
sult of  such  a  combined  habit  would  be  entering  the  door 
next  to  the  end  door.  The  problem  of  entering  always 
the  middle  door  of  those  open  brings  us  closer  to  the  use 
of  memory  ideas.  An  animal  that  had  solved  this  problem 
would,  on  being  confronted  with  the  series  of  doors,  find 
itself  in  an  attitude  representing  a  balance  between  the 
impulse  of  turning  to  the  right  and  that  of  turning  to  the 
left.  "Middleness"  means  a  slight  impulse  to  turn  in 
one  direction,  offset  by  an  equal  impulse  to  turn  in  the 
other  direction.  Now  the  characteristic  by  which  this 
situation  differs  from  the  other  situations,  involved  in  the 
simpler  problems,  is  that  the  animal  must  not  move  at  once, 
but  must  wait  and  assume  the  balanced  attitude  before 
moving.  In  the  case  of  the  other  problems,  he  can  start 
off  immediately.  Here  an  attitude  must  be  revived  before 
there  is  any  actual  movement.  Just  as  in  the  Delayed 
Reaction  Method  success  means,  if  the  animal  moves 
during  the  interval  of  delay,  that  it  is  able  to  revive  an  inner 
attitude  which  means  motion  towards  the  light,  so  here 
success  means  ability  to  revive  an  inner  attitude  which 
means  movement  towards  the  middle,  a  balance  beween 
right  and  left  movements. 


302  The  Animal  Mind 

Further,  what  is  the  difference  between  reviving  such  a 
motor  attitude  at  the  sight  of  a  stimulus,  and  making  an 
ordinary  response  to  a  stimulus,  such  as  any  animal  may 
learn?  The  difference  is  that  in  the  latter  case  an  actual, 
visible  movement  is  made,  while  in  the  former  case  the 
movement  is  internally  anticipated  and  not  externally  visi- 
ble. Such  an  internally  anticipated  movement  is  probably 
always  present  when  in  the  human  consciousness  we  have 
a  memory  idea :  when  I  recall  a  mental  image  of  an  ob- 
ject such  as  a  fork,  I  " internally  anticipate"  the  movements 
of  handling  the  fork.  Whether  the  converse  of  this  prop- 
osition is  also  true,  and  we  invariably  have  memory  ideas 
whenever  we  internally  anticipate  movements,  is  highly 
doubtful,  but  at  least  it  may  safely  be  said  that  an  animal 
which  gives  evidence  of  being  able  to  anticipate  its  own 
movements  has  the  possibility  of  memory  ideas  in  its 
consciousness.  (For  reasons  which  have  been  elsewhere  1 
stated,  the  present  writer  is  inclined  to  think  that  this 
internal  anticipation  of  movements  means  actual  slight  con- 
tractions of  the  muscles  involved  in  performing  the  move- 
ments.) Whenever,  then,  as  in  the  case  of  success  in  the  De- 
layed Reaction  Method  where  the  bodily  position  is  varied, 
in  that  of  inferential  imitation,  and  in  that  of  choosing  al- 
ways the  middle  stimulus,  the  behavior  seems  to  demand 
that  the  movements  shall  be  anticipated  by  the  animal 
which  performs  them,  we  have  evidence  in  favor  of  the 
memory  idea. 

§  78.   Conditions  Favoring  the  Development  of  Memory 

Ideas 

An  important  condition  of  an  animal's  ability  to 
anticipate  its  movements,  to  "know  beforehand"  what  it 

1  Movement  and  Mental  Imagery.     New  York,  1916. 


Modification  by  Experience  303 

is  going  to  do,  is  obviously  the  ability  to  keep  from  actually 
reacting  on  the  instant  when  the  stimulus  acts.  To  recall 
a  memory  idea,  to  anticipate  by  slight  and  invisible  move- 
ments the  response  one  is  going  to  make,  implies  waiting 
a  brief  interval  at  least  before  making  it  in  full.  Now 
the  development  of  sense-organs  which  can  receive  stimuli 
coming  from  a  distance  is  an  absolutely  necessary  prereq- 
uisite for  the  safety  of  delaying  reaction.  An  important 
difference  exists  between  the  stimuli  from  objects  directly 
in  contact  with  an  organism's  body,  such  as  in  our  own 
experience  give  rise  to  touch,  temperature,  pain  and  taste 
sensations,  and  those  which  proceed  from  objects  at  a 
distance,  such  as  light,  sound,  and  odors.  This  differ- 
ence consists  in  the  fact  that  the  former  have  a  more  direct 
and  instant  effect  upon  the  organism's  welfare,  and  in 
consequence  demand  more  rapid  reaction  than  the  latter. 
A  stimulus  in  immediate  contact  with  an  animal's  body 
may  have  a  harmful  or  beneficial  influence  at  the  moment 
of  its  impact ;  it  may  be  food  to  be  seized  or  an  enemy  to 
be  escaped,  and  the  seizing  or  escaping  must  be  done  on 
the  instant ;  on  the  other  hand,  if  an  animal  possesses  the 
power,  belonging  in  an  increasing  degree  to  animals  as  we 
go  up  the  scale,  of  reacting  to  influences  proceeding  from 
objects  still  at  a  distance,  it  may  safely  delay  its  reaction 
when  the  stimulus  is  given.  The  danger  is  not  so  imminent, 
the  food  is  not  yet  within  reach ;  the  full  motor  response 
to  stimulation  may  be  suspended  for  a  short  interval 
without  imperiling  the  life  interests  of  the  animal.  Thus 
one  condition  for  the  development  and  use  of  memory 
ideas  is  the  evolution  of  sense-organs  for  the  reception  of 
stimuli  at  a  distance.  This  idea  was  first  suggested  by  the 
writer  in  1904  (755) ;  a  similar  conception,  developed 
from  the  neurological  standpoint,  appears  in  Sherrington's 


304  The  Animal  Mind 

"The  Integrative  Action  of  the  Nervous  System"  (68 1, 
pp.  324  ff.).  Sherrington  proposes  the  term  "distance 
receptors"  for  those  receptive  organs  "which  react  to 
objects  at  a  distance,"  and  declares  that  "the  distance 
receptors  contribute  most  to  the  uprearing  of  the  cere- 
brum." The  most  important  significance  of  the  power  to 
act  in  response  to  distant  objects  Sherrington  finds  to  be 
that  it  allows  an  interval  for  preparatory  adjustment,  "for 
preparatory  reactive  steps  which  can  go  far  to  influence  the 
success  of  attempts  either  to  obtain  actual  contact  or  to 
avoid  actual  contact  with  the  object."  That  these  pre- 
paratory steps  may  also  involve  the  germ  of  the  memory 
image  is  clearly  suggested  by  Sherrington.  "We  may 
suppose,"  he  says,  "that  in  the  time  run  through  by  a 
course  of  action  focussed  upon  a  final  consummatory  event, 
opportunity  is  given  for  instinct,  with  its  germ  of  memory, 
however  rudimentary,  and  its  germ  of  anticipation,  how- 
ever slight,  to  evolve  under  selection  that  mental  extension 
of  the  present  backward  into  the  past  and  forward  into 
the  future  which  in  the  highest  animals  forms  the  prerog- 
ative of  more  developed  mind.  Nothing,  it  would  seem, 
could  better  insure  the  course  of  action  taken  in  that  in- 
terval being  the  right  one  than  memory  and  anticipatory 
forecast"  (p.  332). 

Secondly,  if  memory  ideas  depend  on  the  anticipation 
of  movements,  during  the  delay  between  stimulus  and 
full  response,  an  important  condition  of  their  variety  and 
free  use  is  the  ability  of  the  animal  to  perform  a  great  variety 
of  movements,  and  especially  of  movements  other  than  those 
of  locomotion.  Locomotion  gets  an  animal  into  difficulties 
and  rescues  it;  movements  of  locomotion  are  of  the  first 
practical  importance.  But  they  have  not  a  great  deal 
of  variety.  It  is  not  merely  a  coincidence  that  the  best 


Modification  by  Experience  305 

evidences  of  memory  ideas  should  appear  in  animals 
which  like  the  raccoon  and  the  monkey  are  dexterous, 
able  to  use  their  paws  for  movements  more  complex  and 
refined  than  those  of  locomotion.  The  supreme  develop- 
ment of  ideas  comes  in  the  mind  of  the  animal  which  has 
not  merely  hands,  but  vocal  organs,  so  that  an  infinite 
variety  of  delicate  and  complicated  movements  can  be 
anticipated,  and  can  form  the  basis  of  memory  ideas. 

Thirdly,  one  of  the  conditions  of  the  anticipation  of  a 
movement  appears  to  be  attention  to  it  when  it  is  originally 
performed.  In  order  to  remember  a  movement,  we  must 
have  paid  attention  to  the  sensations  which  its  performance 
occasions,  to  the  way  it  feels  to  make  the  movement.  And 
one  condition  for  attention  to  the  way  a  movement  feels 
is  being  comparatively  safe  from  external  dangers  when 
the  movement  is  made.  An  animal  under  ordinary  condi- 
tions of  wild  life  has  very  little  attention  to  spare  for  his 
own  movements.  It  would  thus  seem  as  though  one 
requirement  which  must  be  fulfilled  if  anticipated  move- 
ments are  to  play  an  important  part  in  a  creature's  experi- 
ence were  that  the  animal  should,  for  a  time  at  least,  be 
set  free  from  the  pressure  of  the  practical  hand-to-hand 
struggle  for  the  means  of  existence,  and  thus  enabled  in 
safety  to  attend  to  its  own  movement  sensations.  Animal 
play,  at  first  thought,  offers  an  instance  of  such  liberation 
from  practical  necessities.  But  as  Groos  has  shown, 
animal  play  is  not  so  unpractical  as  it  looks  (270).  It  is 
simply  the  exercise  of  the  same  instincts  upon  which  in 
other  circumstances  the  animal's  welfare  depends.  The 
attention  is  absorbed  in  external  objects  quite  as  much 
in  play  as  in  the  actual  chase  or  warfare.  The  kitten 
watches  the  string,  for  which  she  has  no  practical  use, 
as  intently  as  she  watches  the  bird  for  which  she  does 


306  The  Animal  Mind 

have  a  practical  use ;  the  dogs  rolling  over  and  over  each 
other  are  nearly  as  absorbed  in  each  other's  movements 
as  if  they  were  in  deadly  combat. 

That  relief  from  practical  necessity  which  will  serve  the 
purpose  we  are  considering  is  to  be  found  not  in  play,  but 
in  infancy.  If  a  creature  spends  the  period  during  which 
its  nervous  system  is  undergoing  most  rapid  development 
in  a  state  of  complete  shelter  and  protection  from  external 
danger,  with  all  its  vital  needs  supplied,  then  the  nervous 
energy  which  under  other  conditions  would  be  expended 
in  the  processes  underlying  attention  to  external  stimuli 
is  free  to  be  so  devoted  that  attention  will  be  directed 
toward  the  creature's  inner  experiences.  The  human 
baby,  while  he  may  be  interested  in  lights  and  sounds,  in 
external  impressions,  does  not  need  to  be  alert  and  watchful 
lest  he  miss  his  dinner  or  be  dined  on  himself ;  his  atten- 
tion is  free  to  be  expended  on  his  own  movement  experiences 
as  well  as  on  anything  else.  That  young  children  do  go 
through  a  stage  of  intense  interest  in  the  sensations  result- 
ing from  their  own  movements  is  a  fact  made  clear  from 
many  observations.  The  curious  period  of  "  self  -imita- 
tion" in  the  child  when  it  repeats  for  an  indefinite  period 
the  same  movement  or  sound,  over  and  over  again  (14), 
is  very  likely  a  period  of  vivid  attention  to  movement  sen- 
sations. 

That  the  prolonged  period  of  human  infancy  is  of  advan- 
tage to  the  intellectual  life  of  man  because  it  means  plas- 
ticity, the  absence  of  fixed  instincts  that  would  take  the 
place  of  acquisition  by  individual  experience,  was  first 
pointed  out  by  Fiske  (227).  But  quite  as  important  is 
the  fact  that  in  prolonged  infancy  we  have  the  opportunity 
for  acquiring  the  habit  of  that  attention  to  our  own  move- 
ments which  is  the  prerequisite  for  anticipated  movements. 


Modification  by  Experience  307 

There  are,  as  we  have  seen,  various  ways  of  learning  by 
experience  —  slow  ways  that  do  not  involve  ideas,  and 
the  rapid  way  that  does.  The  great  advantage  of  man 
over  most  of  the  lower  animals  is  not  so  much  in  the  fact 
as  in  the  method  of  his  learning.  One  of  the  most  vital 
meanings  of  the  long  period  of  helplessness  and  dependence 
constituting  human  infancy  lies  in  the  fact  that  by  reliev- 
ing from  the  necessity  of  attending  exclusively  to  external 
objects,  it  renders  possible  attention  to  the  sensations 
resulting  from  movement;  and  thus,  by  supplying  an 
essential  condition  for  the  anticipated  movements,  it  opens 
the  way  for  the  control  of  movement  through  ideas. 

§  79.  Some  Alleged  Instances  of  Remarkable  Mental  Powers 
in  Animals 

All  of  the  experimental  evidence  which  we  have  examined 
indicates  that  even  in  the  cleverest  animals  intellectual 
ability  falls  far  short  of  that  demonstrated  by  rather  dull 
human  beings.  But  a  few  years  ago  in  Germany  the 
hypothesis  was  advanced  that  the  minds  of  such  animals 
as  horses  and  dogs  are  really  quite  on  a  par  with  those  of 
human  beings;  their  apparent  deficiencies  being  due  to 
the  fact  that  we  have  never  learned  how  to  educate  and 
communicate  with  animals.  In  1901  a  Berlin  gentleman, 
Herr  von  Osten,  began  training  a  five-year  old  horse  named 
Hans  to  answer  arithmetical  questions  by  tapping  with 
his  hoof  on  the  ground.  Taps  with  the  right  hoof  meant 
units,  taps  with  the  left  hoof  meant  tens.  Later,  an  al- 
phabetic system  was  constructed  on  a  numerical  chart: 
the  letter  a,  for  example,  being  found  in  the  vertical  column 
numbered  3  and  the  horizontal  column  numbered  2,  tap- 
ping three  times  with  the  left  hoof  and  twice  with  the  right 


308  The  Animal  Mind 

meant  a.  Thus  Hans  was  trained  to  answer  questions 
other  than  those  concerned  with  numbers.  He  showed 
ability  to  do  so  with  seeming  intelligence,  and  to  work 
arithmetical  problems.  He  was  examined  successively 
by  two  commissions,  and  a  psychologist  on  the  second 
commission,  Pfungst,  apparently  solved  the  mystery  of 
Hans's  behavior  by  showing  that  the  person  who  put  the 
questions  to  the  horse  made  unintentionally  a  slight  move- 
ment of  the  head  when  the  proper  number  of  taps  had 
been  given,  and  that  when  such  movements  were  inten- 
tionally made,  the  horse  responded  to  them.  So  the  matter 
rested,  with  the  simple  solution  that  the  horse  had,  instead 
of  really  thinking,  merely  reacted  to  involuntary  signals. 
After  the  death  of  Herr  von  Osten,  Hans  came  into  the 
possession  of  Herr  Krall,  a  business  man  of  Elberfeld,  who 
was  not  satisfied  with  Pfungst's  explanation,  and  besides 
continuing  the  education  of  Hans,  trained  several  more 
horses,  the  most  gifted  of  which  were  two  Arabians  named 
Muhammed  and  Zarif.  In  two  weeks  Muhammed  learned 
to  add  and  to  subtract ;  he  passed  in  three  days  from  multi- 
plication and  division  to  the  use  of  fractions ;  he  acquired 
remarkable  skill  in  the  extraction  of  square  and  cube  roots, 
and  finally  he  as  well  as  his  fellow  pupil  began  to  offer 
original  observations.  These  performances  occurred  even 
when  the  horses  were  prevented  from  seeing  any  one.  Much 
the  same  sort  of  phenomena  are  reported  in  the  case  of 
Rolf,  the  Mannheim  dog.  The  most  recent  reports  of  the 
Elberfeld  horses  are  less  enthusiastic,  and  even  claim  fraud, 
although  not  on  the  part  of  Herr  Krall,  whose  disinterested- 
ness seems  accepted.  It  is  impossible  to  determine  just 
what  cues  are  responded  to  by  these  animals  in  their  per- 
formances, but  aside  from  all  the  negative  weight  of  the 
evidence  obtained  under  exact  experimental  conditions  on 


Modification  by  Experience  309 

other  animals  (it  will  be  recalled  that  the  horse  was  the 
stupidest  of  all  Hamilton's  subjects),  certain  indications 
point  clearly  away  from  the  possibility  that  the  horses 
are  really  mathematical  geniuses,  (i)  They  learn  too 
quickly  to  allow  of  their  understanding.  A  gifted  human 
being  could  not  acquire  so  fast  a  real  apprehension  of 
mathematical  relationships.  (2)  They  take  no  longer  for 
hard  problems  than  for  easy  ones.  (3)  They  begin  tapping 
without  even  glancing  at  the  problem  written  on  the 
board.  (4)  The  character  of  the  mistakes  they  make  is 
not  that  of  the  mistakes  of  a  real  calculator :  very  common 
errors  are  reversals  of  the  figures,  thus  27  for  72,  or  errors 
of  one  unit,  as  21  instead  of  22.  These  are  errors  which 
might  easily  be  made  if  the  two  forefeet  were  confused  in 
the  tapping,  or  if  the  tapping  stopped  a  little  too  soon  or 
not  quite  soon  enough.  They  are  not  real  arithmetical 
errors,  such  as  forgetting  to  carry  a  figure  over  from  one 
column  to  another,  for  instance.  (4)  No  really  satisfactory 
results  have  been  reported  when  no  one  present  knew  the 
correct  answer.  On  the  whole,  the  phenomena  do  not 
present  themselves  with  such  authority  as  to  compel  a 
revision  of  our  whole  conception  of  the  animal  mind  (125, 
126,  200,  278,  383,  459,  460,  489,  511,  577  a,  653). 

§  80.   Certain  Influences  Affecting  Learning 

We  may  conclude  our  study  of  the  modification  of  con- 
scious processes  by  individual  experience  with  a  brief  sum- 
mary of  some  incidental  factors  which  affect  the  learning 
process,  (i)  The  age  of  the  animal  has  an  influence  upon 
its  ability  to  learn.  Watson  (766)  compared  the  ability 
of  young  white  rats  with  that  of  mature  animals  in  the 
learning  of  puzzle-box  and  maze  habits.  He  was  espe- 


310  The  Animal  Mind 

daily  interested  in  testing  Flechsig's  theory  that  learning 
depends  upon  the  presence  of  medullated  fibres  in  the  cen- 
tral nervous  system.  The  theory  was  unconfirmed,  for 
such  medullation  is  highly  imperfect  in  the  rat  at  twenty- 
four  days  of  age,  yet  at  this  age  Watson's  rats  learned  a 
labyrinth  more  quickly  than  did  the  adults.  The  rat 
belongs  to  the  class  of  animals  that  are  born  unable  to 
care  for  themselves,  and  before  those  observed  by  Watson 
had  reached  the  age  of  twelve  days,  they  were  unable  to 
find  their  way  by  a  simple  maze  path  back  to  the  mother. 
The  superiority  of  young  rats  over  adults  in  learning  a 
maze  path  is  apparently  due  to  their  greater  activity; 
they  make  more  useless  movements,  and  in  solving  a  puzzle 
box  they  are  at  a  disadvantage  as  compared  with  their 
elders. 

Allen's  (4)  work  on  the  guinea  pig  was  intended  for  com- 
parison with  Watson's  study,  because  the  guinea  pig  comes 
into  the  world,  not  helpless  like  the  baby  rat,  but  well 
equipped  on  both  the  sensory  and  motor  sides.  In  the 
labyrinth  tests  the  mother  was  put  at  the  end  of  the  maze, 
and  the  sight  and  smell  of  her  were  supposed  to  serve  as 
the  stimulus  to  activity.  Before  the  young  animals  reached 
the  age  of  two  days  they  did  not  succeed  in  learning  a 
comparatively  simple  path,  but  at  that  age  they  did  learn 
it,  and  proved  the  fact  when  the  wire  netting  box  in  which 
they  were  placed  was  turned  about,  by  pushing  at  the 
place  where  the  opening  had  been.  At  three  days  they 
learned  a  more  complex  maze,  and  appeared  to  possess  the 
learning  capacity  of  adults. 

Yerkes  (821)  found  that  the  dancing  mouse  at  one  month 
old  learns  a  black-white  discrimination  faster  than  an  older 
mouse.  From  one  to  seven  months  of  age  there  is  a  de- 
crease in  learning  speed ;  from  seven  to  ten  months,  an 


Modification  by  Experience  311 

increase.  The  power  to  discriminate  appears  to  be  better 
in  younger  mice;  the  power  to  associate  better  in  older 
ones.  Thus  the  superiority  of  the  younger  animals  is 
rather  in  speed  of  sense  perception  and  movement  than  in 
real  learning  ability. 

Hubbert  (345)  has  confirmed  the  statement  that  young 
rats  learn  the  maze  more  rapidly  than  older  ones.  More- 
over, she  finds  that  in  the  younger  animals,  the  most  rapid 
stage  of  the  learning  occurs  at  an  earlier  point. 

(2)  The  sex  of  the  learner  may  have  some  effect  on  the 
learning,  although  no  very  definite  differences  have  thus  far 
appeared.     Yerkes  (821)  reports  that  young  male  dancing 
mice  learn  faster  than  females,  and  that  females  from  four 
to  ten  months  of  age  learn  faster  than  males.     Hubbert 
(345)  states  that  except  in  the  cases  of  very  young  and  very 
old  rats,  males  learn  more  readily  than  females ;  the  absolute 
time  of  running  the  maze  is  however  shorter  for  females. 

(3)  The  number  of  trials  a  day  affects  the  speed  of  the 
learning.     Yerkes   (820)    found   that  the  dancing  mouse 
learned  a  white-black  discrimination  in  fewer  trials  the 
smaller  the  number  of  trials  a  day.     Ulrich  (738)  has  shown 
that  the  white  rat  learns  a  puzzle-box  habit  or  a  maze 
habit  in  fewer  trials  if  one  trial  is  given  a  day  than  it  requires 
if  either  three  or  five  trials  are  given  a  day.     Apparently 
even  better  results  are  secured  by  one   trial  every  third 
day.    The  same  principle  appeared  to  hold  when  several 
problems   were   being   learned   at   once.     This   principle, 
known  as  that  of  distributed  repetitions,  has  long  been 
recognized  in  human  memorizing,   although  we  do  not 
know  the  explanation  for  it.     But  learning  is  always  more 
economically  secured  if  intervals  of  time  are  allowed  to 
elapse  between  repetitions. 

(4)  The  learning  of  one  habit  may  influence  the  later 


The  Animal  Mind 

'  acquisition  of  other  habits.  Yerkes  (820)  reports  that 
dancing  mice  which  have  learned  one  maze  learn  another 
one  more  readily  than  those  which  have  had  no  previous 
training.  Richardson  (634)  finds  previous  experience  a 
help  also  to  the  rat :  experienced  animals  were  more  sus- 
ceptible to  stimuli  and  showed  better  coordination  of  their 
activities.  Hunter  (349),  on  the  other  hand,  found  that 
pigeons  which  had  learned  one  maze  were  delayed  in  learn- 
ing a  second  one,  and  Yoakum  (832)  reports  a  similar  condi- 
tion in  the  learning  of  puzzle-boxes  by  squirrels :  the  older 
habits  interfere  with  the  acquisition  of  the  newer  ones. 
Hunter  and  Yarbrough  (355)  conclude  from  experiments  on 
establishing  auditory  associations  in  white  rats  that  a 
formed  habit  interferes  with  the  formation  of  a  new  one,  but 
that  the  new  habit  does  not  react  unfavorably  upon  the  old 
one.  This  has  been  found  true  of  human  memorizing  also. 
Probably,  when  an  animal  seems  to  learn  a  new  habit  better 
because  of  having  previously  formed  a  different  habit,  the 
advantage  is  merely  in  the  fact  that  it  has  become  used  to 
being  experimented  upon,  to  the  experimental  situation. 

(5)  The  differences  in  individual  ability  among  animals 
are  marked.  We  are  inclined  to  think  of  all  the  animals 
of  a  certain  species,  especially  if  it  be  a  species  far  removed 
from  man,  as  equally  gifted,  but  it  is  quite  possible  that 
among  ants  and  earthworms  there  are  geniuses  and  dunces. 
Turner  (729,  730)  reports  striking  individual  variations  in 
the  behavior  of  cockroaches  learning  a  maze;  two  of  the 
rats  tested  by  Small  (685)  with  puzzle-boxes  never  learned 
to  get  into  the  boxes,  but  merely  profited  by  the  activity 
of  their  more  gifted  companions.  Wodsedalek  (795) 
gives  a  delightful  account  of  a  specially  talented  Mayfly. 
Practically  every  experimenter  reports  similar  individual 
variations. 


CHAPTER  XII 

SOME  ASPECTS  OF  ATTENTION 

THE  student  absorbed  in  reading  "does  not  hear"  an  ap- 
proaching footstep.  That  is,  a  stimulus  which  would  under 
other  circumstances  produce  an  effect  loses  a  great  part  of 
its  influence  because  of  the  fact  that  another  stimulus  is  al- 
ready upon  the  field.  This  other  stimulus  need  not  be  more 
intense,  that  is,  need  not  involve  more  physical  energy, 
than  the  one  which  is  ignored.  It  does  not  win  the  victory 
by  a  mere  swamping  of  its  rival  through  its  superior  quan- 
tity. A  man  may  walk  along  city  streets,  his  eyes  and  ears 
bombarded  with  brilliant  lights  and  loud  sounds,  and  yet 
the  centre  of  his  consciousness  may  be  a  train  of  ideas, 
representing  in  their  physical  accompaniment  in  his  cortex 
a  quantity  of  energy  insignificant  compared  with  that  of  the 
external  stimuli  pouring  in  upon  him.  Psychologists  com- 
monly express  this  fact  by  saying  that  while  the  strength  of 
a  stimulus  conditions  the  intensity  of  the  mental  process 
accompanying  it,  the  clearness  of  that  process  depends 
upon  attention. 

§  81.    The  Interference  of  Stimuli 

Attention,  then,  is  the  name  given  to  a  device,  whatever 
its  nature,  whereby  one  stimulus  has  its  effectiveness  in- 
creased over  that  of  another  whose  physical  energy  may 
be  greater.  What  happens  in  the  simpler  forms  of  animal 
life  when  two  stimuli,  requiring  different  reactions,  operate 

313 


314  The  Animal  Mind 

simultaneously?  We  may  quote  from  Jennings  the  facts 
about  Paramecium.  "If  the  animal  is  at  rest  against  a 
mass  of  vegetable  matter  or  a  bit  of  paper,  .  .  .  and  it 
is  then  struck  with  the  tip  of  a  glass  rod,  we  find  that  at  first 
it  may  not  react  to  the  latter  stimulus  at  all."  "A  strong 
blow  on  the  anterior  end  causes  the  animal  to  leave  the  solid 
and  give  the  typical  avoiding  reaction."  "If  specimens 
showing  the  contact  reaction  are  heated,  it  is  found  that  they 
do  not  react  to  the  heat  until  a  higher  temperature  is  reached 
than  that  necessary  to  cause  a  definite  reaction  in  free-swim- 
ming specimens."  "  On  the  other  hand,  both  heat  and  cold 
interfere  with  the  contact  reaction.  Paramecia  much  above 
or  much  below  the  usual  temperature  do  not  settle  against 
solids  with  which  they  come  in  contact,  but  respond  in- 
stead by  a  pronounced  avoiding  reaction."  "  Specimens  in 
contact  with  a  solid  react  less  readily  to  chemicals  than  do 
free  specimens.  .  .  .  On  the  other  hand,  immersion  in 
strong  chemicals  prevents  the  positive  contact  reaction." 
"The  contact  reaction  may  completely  prevent  the  reaction 
to  gravity,"  and  to  water  currents.  It  also  modifies  the  re- 
action to  the  electric  current.  While  a  part  of  the  influence 
exerted  by  the  contact  reaction  on  other  responses  may  be 
purely  physical,  due  to  the  fact  that  an  actual  secretion  of 
mucus  may  occur  whereby  the  animal  "sticks  fast"  to  the 
solid,  yet  this  alone  does  not  explain  the  facts,  for  the  cilia 
that  are  not  attached  do  not  behave  normally.  The 
reaction  to  gravity  regularly  yields  whenever  opposed  to 
the  action  of  any  other  stimulus  (378,  pp.  92  ff.). 

Sometimes  the  action  of  one  form  of  stimulation  merely 
affects  the  form  of  the  response  to  another,  as  in  the  case 
where  abnormal  temperature  causes  the  avoiding  instead 
of  the  positive  reaction  to  be  given  to  solids.  In  other  cases, 
reaction  to  one  of  the  stimuli  is  suppressed  or  weakened. 


Some  Aspects  of  Attention  315 

The  facts  suggest  that  the  influential  stimulus  is  either  the 
one  that  is  on  the  field  first  (the  contact  reaction  may  prevent 
response  to  temperature,  or  abnormal  temperature  may 
modify  the  contact  reaction),  or  the  one  that  is  the  more 
important  (gravity  yields  always  to  other  stimuli). 

In  some  higher  animals  the  effects  of  interference  of 
stimuli  have  been  noted.  The  earthworm  will  not  respond 
to  light  if  feeding  (171)  or  mating  (327).  In  the  turbellarian 
Conwluta  roscojffensis  light  is  victorious  over  heat  in  deter- 
mining reaction.  The  animals  in  their  positively  photo- 
tropic  phase  will  remain  in  the  heated  light  end  of  a  vessel 
until  they  perish.  Light  and  gravity  are  more  nearly 
balanced  in  their  effects.  Convoluta  is  negatively  geo- 
tropic,  yet  if  the  brightest  region  is  below  the  surface, 
the  animals  will  go  there.  But  if  this  region  is  only  a  little 
brighter  than  the  surface,  they  will  stay  at  the  surface, 
gravity  dominating  (253).  The  sea-urchin  shows  in  its 
behavior  a  somewhat  similar  relation  between  mechanical 
and  chemical  stimulation.  If  weak  acid  is  dropped  into 
the  water  containing  specimens  of  Arbacia,  their  spines 
begin  to  interlace.  A  slight  shaking  will  restore  them  to 
the  normal  position,  but  if  more  acid  be  added,  no  mechan- 
ical stimulation  will  overcome  the  effect  of  the  chemical 
(734)-  Various  facts  concerning  the  interrelations  of 
gravity  and  light  as  stimuli  have  been  noted  in  Chapter 
IX.  A  very  interesting  case  of  the  suppression  of  one 
reaction  by  another  is  reported  by  Holmes  in  his  obser- 
vations on  the  water  insect  Ranatra.  The  positive  response 
of  this  insect  to  light,  very  precise  and  striking,  may  be 
wholly  suspended  when  the  animal  is  feeding,  when  a  num- 
ber of  individuals  are  collected,  when  the  insect  stops  to 
clean  itself,  or  even  "by  the  sudden  appearance  of  a  large 
object  in  the  field  of  vision,"  behavior  which  is  strongly 


316  The  Animal  Mind 

suggestive  of  the  " distraction  of  attention"  in  a  human 
being  (335).  Holmes  (337)  also  observed  that  the  fiddler 
crab,  although  it  ordinarily  moves  towards  the  light,  would 
run  away  from  a  moving  light,  fear  overcoming  positive 
phototropism.  Roubaud,  in  a  study  of  the  behavior  of 
some  species  of  flies  that  live  on  the  seashore,  feeding  on 
dead  fish  and  the  like,  says  that  they  will  abandon  the 
"head  on"  position  which  they  regularly  assume  toward 
the  wind,  if  attracted  by  the  odor  of  food  (646). 

Wherever  we  find  that  one  class  of  stimuli  regularly 
yields  to  another  if  the  two  act  together,  it  is  safe  to  assume 
that  the  prepotent  stimulus  is  more  important  to  the  organ- 
ism's welfare  than  the  vanquished  one.  And  while  we  can- 
not without  more  ado  call  such  cases  of  the  interference  of 
stimuli  as  are  found  in  very  simple  animals  cases  of  atten- 
tion, and  ascribe  to  their  psychic  accompaniment  all  the 
characteristics  of  attention  as  a  feature  of  our  own  expe- 
rience, yet  we  may  assert  that  they  have  in  common  with 
attention  the  significance  of  being  a  device  to  secure  reaction 
to  the  most  vitally  important  of  several  stimuli  acting  at  once 
upon  the  organism, 

§  82.   Methods  of  securing  Prepotency  of  vitally  Important 

Stimuli 

An  inanimate  object  acted  upon  by  several  forces  at  once 
is  determined  in  its  motion  by  their  relative  intensity.  Con- 
ceivably, an  extremely  simple  form  of  animal  life,  when 
subjected  to  two-  stimulations  acting  together,  would  also 
respond  in  a  way  answering  precisely  to  the  relative  strength 
of  the  two.  It  is  easy  to  see  what  would  be  the  disadvantage 
of  such  a  state  of  affairs  for  the  animal.  The  weaker  of 
the  two  stimuli  might  be  of  far  greater  significance  for 


Some  Aspects  of  Attention  317 

organic  welfare  than  the  stronger.  For  example,  it  would 
often  be  important  that  an  animal  should  be  able  to  respond 
to  a  very  faint  food  stimulus  rather  than  to  any  of  the 
stronger  forces  acting  upon  it.  Evidently  a  prime  need  of 
animal  life  is  some  arrangement  whereby  weak  but  im- 
portant stimuli  shall  be  given  the  preference  in  determining 
reaction  over  stronger  but  less  vitally  necessary  ones. 
Sense  organs  are  one  such  device.  The  comparatively 
slight  amount  of  chemical  energy  coming  from  a  bit  of 
food  may  have  its  effectiveness  for  the  nervous  system 
greatly  increased  through  its  reception  by  a  structure 
adapted  to  use  the  whole  of  it  to  advantage.  Light  stimu- 
lation involves  a  quantity  of  energy  that  is  insignificant 
in  comparison  with  the  grosser  forces  acting  on  an  organism  ; 
yet  falling  on  the  retina,  the  energy  is  economized  and 
magnified  through  the  stored-up  chemical  forces  it  sets 
free.  Thus  a  weak  stimulus  may  by  a  sense  organ  be  made 
powerful  to  determine  reaction.  Another  arrangement 
to  the  same  effect  is  the  peculiarity  of  the  nervous  system 
whereby,  through  an  arrangement  akin  to  the  summation 
of  faint  stimuli,  a  moving  stimulus,  one  acting  successively 
upon  neighboring  points  of  a  sensitive  surface,  produces  an 
effect  disproportionate  to  its  intensity.  A  moving  stimulus  is 
a  vitally  important  stimulus ;  it  means  life,  and  hence  may 
mean  food  or  danger.  The  response  to  it  is  in  most  cases 
adapted  rather  to  its  importance  than  to  its  physical 
strength.  A  third  arrangement  for  the  securing  of  reaction 
to  vitally  important  stimulation  lies  in  the  existence  of 
preformed  connections  in  the  nervous  system,  which  bring 
it  about  that  the  path  of  the  excitation  produced  by  one  stimu- 
lus is  clear  to  the  motor  apparatus,  while  that  of  another  is 
closed.  Reactions  of  this  sort  we  call  instinctive.  The 
nesting  bird  responds  to  the  sight  of  building  material  rather 


318  The  Animal  Mind 

than  to  that  of  objects  offering  equally  strong  stimulation 
to  the  optic  nerve ;  the  cat  sits  at  the  mouse  hole,  the  parent 
animal  responds  to  the  faintest  cry  of  the  offspring,  because 
these  stimuli  have  the  right  of  way  by  virtue  of  inherited 
nervous  connections. 

Finally,  a  weak  stimulus  may  determine  reaction  and  be 
victorious  over  a  stronger  one  because  of  nervous  pathways 
formed  through  the  individual's  own  experience.  The  conse- 
quences of  reaction  to  it  in  the  individual's  past  may  operate 
to  secure  reaction  to  it  in  the  future.  To  the  cat  in  a  puzzle 
box,  the  string  that  must  be  pulled  to  let  it  out  offered 
originally  no  stronger  stimulus  to  action  than  any  other  ob- 
ject in  sight ;  but  after  sufficient  experience  the  string  comes 
to  dominate  the  situation  and  determine  the  cat's  behavior. 
If  the  experience  of  consequences  is  slowly  acquired,  by 
many  repetitions,  the  process  of  reacting  to  an  object 
originally  indifferent  may  be  unaccompanied  by  any  ideas 
of  the  consequences  of  such  reaction.  If  it  is  rapidly 
acquired,  we  know  that  we  human  beings  at  least  accom- 
pany our  reactions  by  calling  up  the  results  of  our  past 
reactions  in  the  form  of  memory  ideas. 

§  83.    The  Peculiar  Characteristics  of  Attention  as  a  Device 
to  Secure  Prepotency 

We  have  suggested  that  attention  is  a  means  of  securing 
reaction  to  the  vitally  important  stimuli  acting  upon  an 
organism.  Does  reaction  to  a  stimulus  always  mean  atten- 
tion to  the  sensation  accompanying  that  stimulus  ? 

This  question  may  best  be  answered  by  examining  the 
characteristics  of  the  attention  process  as  we  know  it.  In 
attention,  the  details  of  the  object  attended  to  become  clear 
and  distinct.  That  is,  attention  is  a  state  where  discrimina- 


Some  Aspects  of  Attention  319 

tion  is  improved.  Further,  attention  involves  varying  de- 
grees of  effort,  and  these  are  marked  by  varying  intensity  of 
certain  bodily  processes.  Attention  under  difficulties  is 
accompanied  by  a  rigid  position  of  the  body,  by  holding  the 
breath,  and  by  various  muscular  effects,  aside  from  the  pro- 
cesses which,  like  frowning,  are  concerned  with  the  adapta- 
tion of  the  sense  organ  to  receive  an  impression.  These 
general  bodily  effects  of  attention  are  all  such  as  to  suggest 
that  the  body  is  to  be  kept  as  quiet  as  possible  during  the 
attentive  state.  In  other  words,  no  reaction  is  to  be  made 
to  the  object  attended  to  except  such  as  may  be  necessary 
to  allow  its  being  carefully  discriminated  from  other 
objects.  Attention,  in  its  intenser  degrees,  at  least,  seems  to 
involve  a  state  of  suspended  reaction. 

Not  every  case,  then,  of  response  adapted  to  the  vital  im- 
portance of  a  stimulus  is  a  case  that  suggests  as  its  psychic 
aspect  attention  to  the  accompanying  sensation.  When,  for 
example,  a  reaction  of  especial  speed  is  made  to  contact  with 
a  moving  stimulus,  the  speed  of  the  reaction  would  itself 
indicate  that  the  sensations  produced  are  not  attended  to. 
The  proper  situation  for  attention  would  be  the  situation  in 
which  the  reaction  needs  to  be  suspended  until  the  stimu- 
lus is  fully  discriminated.  Now  such  careful  discrimination 
does  not  appear  to  be  characteristic  of  reactions  that  are 
largely  based  on  inherited  nervous  structures.  Many  facts 
concerning  the  instincts  of  animals,  that  is,  their  inherited 
reactions,  indicate  that  these  are  extremely  rough  adjust- 
ments of  behavior  to  environment  until  refined  by  individual 
experience.  Hudson  observed,  for  example,  that  newly 
born  lambs  on  the  South  American  plains  had  a  tendency  to 
run  away  from  any  object  that  approached  them,  and  to 
follow  any  object  that  receded  from  them.  They  would 
follow  his  horse  for  miles  as  he  rode  along,  and  would  run 


320  The  Animal  Mind 

away  from  their  own  mothers  when  the  latter  moved  toward 
them.  He  explained  this  as  adapted  to  the  fact  that  ordi- 
narily their  first  duty,  on  making  their  appearance  in  the 
world,  is  to  keep  up  with  the  receding  herd,  while  an  ap- 
proaching object  is  more  likely  to  be  an  enemy  (347). 
Later,  this  rough  adjustment  is  modified ;  they  learn  by 
experience  not  to  run  away  from  their  mothers,  and  not 
to  follow  indiscriminately  any  leader. 

If  it  is  true  that  instinct  unmodified  by  experience  is 
adapted  to  general  rather  than  to  special  features  of  environ- 
ment, it  seems  likely  that  the  phenomena  of  attention  as  we 
know  them  are  found  chiefly  in  connection  with  those  re- 
sponses to  vitally  important  stimulation  which  are  deter- 
mined, in  part,  at  least,  by  the  individual  experience  of 
the  reacting  animal,  for  these  are  the  responses  requiring 
most  careful  discrimination  among  stimuli,  and  the  delay 
of  reaction  until  such  discrimination  has  been  made.1 
Putting  .the  matter  in  a  slightly  different  way,  we  may 
say  that  purely  inherited  responses  can  be  adapted  only  to 
certain  broad,  roughly  distinguished  classes  of  stimuli,  for 
these  alone  are  common  to  the  experience  of  all  members  of 
the  species.  Nothing  but  individual  experience  can  bring 
to  light  the  importance  for  welfare  of  certain  particular 
stimuli,  for  the  significance  of  these  would  vary  with  the 
experience  of  each  individual  animal.  Among  the  lower 
animals,  attention  probably  reaches  its  highest  pitch  where 
the  response  most  needs  to  be  suspended  in  order  that  the 

1  In  this  connection  Franz's  experimental  demonstration  that  the  frontal 
lobes,  long  regarded  as  the  seat  of  the  neural  processes  underlying  atten- 
tion, are  concerned  in  the  functioning  of  recently  learned  reactions,  is  of 
especial  interest.  Franz  found  that  cats  and  monkeys  which  had  been 
trained  to  work  mechanisms  lost  the  power  to  do  so  when  the  frontal  lobes 
were  extirpated,  although  habits  of  older  date,  such  as  responding  to  a  call, 
were  preserved  (237,  238). 


Some  Aspects  of  Attention  321 

stimulus  may  be  fully  discriminated.  The  rabbit  or  wild 
bird  crouching  motionless  close  to  the  ground,  watching 
each  movement  of  a  possible  enemy,  suggests  strongly  to 
our  minds  a  condition  of  breathless  attention.  Whether 
such  an  interpretation  is  the  true  one  depends  very  much, 
I  should  say,  on  the  extent  to  which  past  individual  expe- 
rience has  refined  the  animal's  powers  of  discrimination. 
Mere  " freezing  to  the  spot"  may  be  an  inherited  reaction, 
useful  in  time  of  danger,  but  more  analogous  in  its  psychic 
aspect  to  the  blank  emptiness  of  the  hypnotic  trance  than 
to  alert,  watchful  attention. 

Yet  although,  in  so  far  as  attention  is  a  state  favoring 
discrimination  of  stimuli,  it  is  involved  in  that  part  of  an 
animal's  behavior  which  is  derived  from  individual  expe- 
rience, since  pure  instinct  discriminates  but  roughly ;  in  so 
far  as  it  is  still  one  of  the  devices  for  securing  reaction  to 
stimuli  of  vital  importance,  its  root  must  lie  in  instinct.  No 
object  wholly  unrelated  to  some  fundamental  instinct  can 
hope  to  secure  attention,  for  the  great  classes  of  vitally 
important  stimuli  have  all  of  them  preformed  paths  in  the 
nervous  system  by  which  their  reactions  are  secured.  What 
individual  experience  does  is  to  refine  upon  the  adaptations 
which  instinct  makes  possible ;  to  bring  about  the  connec- 
tion of  certain  stimuli,  originally  indifferent,  with  the  per- 
formance of  an  instinctive  response,  or  to  produce  a  check- 
ing of  the  instinctive  response  when  certain  individual 
peculiarities  of  a  stimulus  that  would  otherwise  call  it 
forth  become  evident.  For  instance,  an  animal  learns  by 
experience  to  come  at  the  call  of  a  human  being  who  feeds 
it ;  the  sound,  originally  without  effect  on  its  reactions,  has 
come  to  be  connected  with  the  nervous  mechanism  of  an 
instinct.  The  chick  pecking  at  small  objects  on  the  ground 
learns  by  experience  to  inhibit  this  instinctive  response  with 


322  The  Animal  Mind 

reference  to  objects  having  certain  peculiarities  originally 
undiscriminated,  but  now  in  some  way  emphasized  through 
painful  circumstances  accompanying  his  previous  encounter 
with  them.  • 

The  most  fundamental  characteristic  of  attention,  then, 
is  perhaps  that  aspect  of  it  which  has  been  called  abstrac- 
tion, the  diminished  effectiveness  of  stimuli  not  attended  to. 
By  virtue  of  this  aspect  we  recognize  that  attention  belongs 
with  instinct  as  being  concerned  in  securing  the  prepotency 
of  vitally  important  stimulation.  On  the  other  hand,  the 
further  characteristic  of  attention ;  namely,  that  it  is  a  state 
of  suspended  reaction  involving  careful  discrimination  of 
stimuli,  suggests  that  its  functioning  is  connected  rather 
with  the  refining  and  modifying  influence  of  individual  expe- 
rience acting  on  instinct,  since  here  alone  do  we  find  delayed 
reaction  and  accurate  stimulus  discrimination. 

The  highest  grade  of  attention,  the  final  triumph  of  vital 
importance  over  mere  intensity  of  stimulation,  is  to  be  found 
where  the  focus  of  attention  is  occupied  by  an  idea  or  train 
of  ideas.  When  a  process  purely  centrally  excited  holds  the 
field  and  makes  the  individual  deaf  and  blind  to  powerful 
external  stimuli  pouring  in  upon  his  sense  organs,  then  he 
is  superior  to  the  immediate  environment  at  least.  This 
form  of  attention  occurs,  probably,  only  when  the  vital  im- 
portance of  the  idea  attended  to  has  been  learned  through 
that  most  rapid  form  of  individual  acquisition  of  experience 
which  involves  the  revival  of  the  past  in  idea.  It  has  been 
called  derived  attention.  The  ideas  attended  to  are  held 
in  the  focus  of  consciousness  and  analyzed  through  the  power 
of  associated  ideas.  The  inventor  holds  to  his  problem,  the 
student  to  his  task,  in  spite  of  distractions,  because  of  the 
consequences  which  he  thinks  of  as  likely  to  result.  It 
seems  unlikely  that  attention  in  this  final  form  occurs  among 


Some  Aspects  of  Attention  323 

the  lower  animals.  While  ideas  are  probably  present  to 
some  extent  in  the  minds  of  the  higher  mammals,  they  are 
hardly  so  far  freed  from  connection  with  external  stimuli 
that  the  animal  can  shut  out  the  world  of  sense  from  its 
consciousness  and  dwell  in  a  world  of  ideas. 


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340  The  Animal  Mind 

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2  A 


354  The  Animal  Mind 

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INDEX   OF  SUBJECTS 


Abstraction,  322. 

Accommodation,  241. 

Acephala,  chemical  sense,  84. 

Actinia,  73,  182,  193,  203,  254,  259. 

Actinobolus,  69. 

Adamsia,  72,  74,  254. 

Adaptation,  sensory,  251  ff.f  256  f. 

Adaptation,  darkness,  149,  152,  165  f., 
168. 

/Eolosoma,  83. 

Age,  effect  on  learning,  309  f. 

Aiptasia,  73,  215,  251,  254. 

Allolobophora,  81  f. 

Amblystoma,  112. 

Amoeba,  38  fT.,  response  to  change  in 
light  intensity,  135  ;  response  to  colors, 
146;  reaction  to  locali/.ed  stimuli, 

173  f- 

Amphibia,  chemical  sense,  in  f. ;  color 
vision,  163  f. 

Amphioxus,  chemical  sense,  109;  color 
vision,  159  f. 

Amphipods,  194. 

Amphithoe,  88. 

Anecdote,  Method  of,  4  ff . 

Anemotropism,  178,  213  f. 

Annelids,  chemical  sense,  81  ff. ;  orienta- 
tion to  gravity,  184  f . ;  hearing,  117; 
color  vision,  147  f . ;  response  to  change 
in  light  intensity,  139  f. ;  251  f. 

Anticipation  of  movements,  302  ff. 

Ants,  taste  in,  91 ;  smell,  94  ff. ;  homing, 
96  ff. ;  recognition  of  nestmates,  100 
ff . ;  hearing,  123  f . ;  color  vision, 
I5S;  perception  of  light  direction, 
220  f. ;  form  perception,  230;  252, 
255  ;  maze  experiments,  273. 

Arachnida,  chemical  sense,  89  f.  See 
Spiders. 

Arenicola,  146. 

Arthropods.  See  Crustacea,  Spiders, 
Insects. 

Associative  memory,  30  f. 


Asterias,  141. 

Attention,  52,  253,  284,  305  f.,  313  ff. 

Attitudes,  revived,  301  f. 

Auditory  cues  in  maze  running,  281  f. 

Axolotl,  260. 

Background,    effect    on    phototropism, 

207. 

Balancing  reactions,  191  f. 
Balanus,  201,  203. 
Bat,  hearing,  134. 
Bees,  attraction  to  flowers,  104  f.,  157  f.J 

homing,  105  ff.,  231,  287;   recognition 

of  hivemates,   107  ff. ;    hearing,   124; 

color  vision,  155  ff. ;   form  vision,  225. 
Behavior,  as  evidence  of  discrimination, 

55  «. 

Behaviorism,  22  ff.,  277. 

Bembex,  234  ff. 

Beroe,  77. 

Binocular  vision,  220,  241  f. 

Birds,  smell  and  taste,  112  f. ;  color 
vision,  165  ff. ;  hearing,  131  f. ;  form 
vision,  226  f. ;  migration,  236  f. ; 
puzzle  box  experiments,  268  f . ;  maze 
experiments,  274  f.;  Multiple  Choice 
Method,  299  f. 

Bispira,  251. 

Blowfly  larva,  159. 

Branchipus,  142,  188,  210. 

Brightness  value  of  colors,  145  ff. 

Bunsen-Roscoe  Law,  195. 

Bursaria,  68. 

Calyptraea,  85. 

Carmarina,  75  f. 

Cat,    113,    134,    i?i,    266,    269  f.,    289, 

292  f. 

Caterpillar,  225. 
Centrostephanus,  141,  252. 
Cereactis,  146. 
Cerianthus,  72,  182. 
Chemical  sense,  63  ff. 


377 


378 


Index 


Chemicals,  effect  on  phototropism,  203. 

Chick,  224,  229  f.,  266,  268. 

Chlamydomonas,  178. 

Choice,  as  evidence  of  mind,  28. 

Chordotonal  organs,  121  ff. 

Chromotropism,  148,  154. 

Cirripedia,  142. 

Clepsine,  139. 

Cockroach,  121,  264,  312. 

Coelenterates,  chemical  sense,  69  ff. ; 
hearing,  116  f. ;  response  to  change  of 
light  intensity,  136  ff. ;  response  to 
color,  146;  geotropism,  182  f.;  re- 
sponse to  motion,  215  f. 

Color  vision,  invertebrates,  144  ff. ; 
amphioxus  and  fish,  159  ff. ;  reptiles 
and  amphibians,  163  ff. ;  birds,  165 
ff . ;  mammals,  169  ff. 

Common  chemical  sense,  no. 

Communication  in  ants,  95  f . 

Compound  eye,  217  ff.,  241  f. 

Continuous  action  theory  of  tropisms, 
194  ff- 

Contrast,  151  f. 

Convoluta,  184,  201,  209,  315. 

Copepods,  89,  189. 

Corymorpha,  71,  182. 

Cow,  291  f. 

Crab,  153,  231,  263,  273. 

Crayfish,  88,  152,  188  f.,  217,  273. 

Cricket,  122  f. 

Crow,  299  f . 

Crustacea,  chemical  sense,  87  ff. ;  hear- 
ing, 118  f. ;  response  to  change  of 
light  intensity,  142;  color  vision, 
149  f . ;  geotropism,  188  f.;  202  f. 
See  Crab,  Crayfish,  Hermit  Crab. 

Ctenophors,  183. 

Cypridopsis,  204. 

Cypris,  204. 

Daphnia,  149  ff.,  194,  201  ff. 

Delayed  reactions,  242  f.,  302  ff.,  318  f. 

Delayed  Reaction  Method,  290  f., 
297  f- 

Density  of  water,  effect  on  phototro- 
pism, 202  f. 

Depth  perception,  237  ff. 

Didinium,  69. 

Difficulties  of  comparative  psychology, 
i  ff. 

Dina,  139. 

Dinetus,  109. 

Direction  of  light,  perception  of,  220  f. 


Direction  theory  of  tropism,  195,  199  f. 
Discrimination,     evidence    of,     54    ff. ; 

acquired  by  punishment,  264  ff. 
Distance  receptors,  303  f. 
Distributed  repetitions,  311. 
Dog,   113,   133  f.,   171,   227  f.,   269  f., 

287  f. 

Dragon  fly,  252. 
Dreaming  in  animals,  287  f. 
Dropping  out  of  movements,  258  ff. 
Dysdera,  221. 
Dytiscus,  91. 

Earthworm,  81  f.,  117,  139,  146  f.,  175, 
198  f.,  223,  263,  273,  3IS- 

Echinaster,  141  f. 

Echinoderms,  chemical  sense,  86;  hear- 
ing, 117;  response  to  change  of  light 
intensity,  141  f. ;  color  vision,  149, 
See  Starfish,  Sea-urchins. 

Elberfeld  horses,  308  f. 

Electric  shock  method,  265. 

Eloactis,  137. 

Emotional  adaptation,  255  f. 

Emotional  aspect  of  maze  running,  284. 

Errors,  order  of  eliminating,  275  f. 

Euglena,  136,  178,  199. 

Evolutionary  writers,  attitude  toward 
animal  mind,  15  f. 

Experiment,  Method  of,  9  ff. 

Fatigue,  60,  252  f.,  255. 

Fear,  effect  on  phototropism,  208. 

Fiddler  crab,  119,  188,  208. 

Firefly,  298. 

Fish,  chemical  sense,   109  ff. ;    hearing, 

126  ff. ;    color  discrimination,  160  ff. ; 

orientation  to  gravity,  190  ff. ;  rheot- 

ropism,  212  ff. ;   distance  perception, 

238;    learning,  259  f.,  263,  273. 
Flatworms,     chemical     sense,     78     ff. ; 

hearing,      117;      photokinesis,      144; 

geotropism,  183;    phototropism,  199; 

223,  249  f. 

Form,  visual  perception  of,  225  ff. 
Frequency,  276. 
Frog,  in  f.,  130  f.,  163  f.,  223,  263,  274. 

Gelasimus,  119,  188,  208. 

Geodesimus,  80. 

Geotropism,  in  Protozoa,  178  ff. ;'"  in 
coelenterates,  182  f. ;  in  planarians, 
183  f. ;  in  mollusks,  185  f. ;  in  spiders 
and  insects,  189  f. ;  in  annelids,  184  f. ; 


Index 


379 


in  echinoderms,  186  f.;   in  Crustacea, 
188  f . ;  in  vertebrates,  190  ff. ;  psychic 
aspect,  192 ;  effect  on  light  reactions, 
209  ff. 
Gonioneraus,  76  f.,  138  f.,  144,  183,  201, 

215- 
Guinea-pig,    310. 

Hans,  Clever,  307. 

Harmful     movements,     dropping     off, 

261  f. 
Hearing,  in  lower  invertebrates,  116  ff., 

in  Crustacea,  118  f. ;  in  spiders,  119  f. ; 

in  insects,  121  ff.;    in  fishes,  126  ff.; 

in  amphibia,  130  f.;   in  reptiles,  131; 

in  birds,  131  f. ;   in  mammals,  132  ff. 
Hedista,  206. 
Heightened  reaction  to  repeated  stimuli, 

246  ff. 
Helix,  84  f  • 

Hermit  crab,  87,  206,  260. 
Hippolyte,  207. 
Homing,  in  ants,  96  ff . ;   230  f . ;  in  bees, 

231  f. ;    in  pigeons,  237. 
Horse,  299,  307  f. 
Hunger,  203,  253,  254  f. 
Hydra,  69  f.,   137,   143,   146,   182,   247, 

249,  251,  256  f. 

Ideas,  33  f.,  47  ff-,  250,  264,  287  ff. 
Image,  visual,  218  ff. 
Imitation,  instinctive,  288  f. 
Imitation,  inferential,  289  ff.,  296  f.,  302. 
Incidental  cues,  response  to,  58. 
Individual  differences  in  learning  ability, 

312. 

Infancy,  value,  306  f. 
Insects,  chemical  sense,  90  ff. ;   hearing, 

121    ff. ;     photokinesis,     144;     color 

vision,    154.  ff. ;    geotropism,    189    f., 

210;  size  perception,  223. 
Instinct,  291,  317. 

Intensity  of  light,  effect  on  tropism,  201, 
Intensity  theory  of  tropism,  195  f. 
Interference  of  habits,  311  f. 
Interference  of  stimuli,  313  ff. 
Interpretation    of    behavior,    methods 

13  ff- 
Invertebrate  eye,  simple,  217. 

Jassa,  203. 

Jellyfish,  75  ff.,  137  f.,  175,  183. 

Kinaesthetic  sensation,  62,  280  ff. 
Kinaesthetic  memory,  100,  282  f.,  300  f 


-abidocera,  201,  204. 

,acrymaria,  69. 

^andmarks,  recognition  of,  286  f. 

..ateral  line  organs,  128  ff. 
Learning,  30  ff.,  245  ff.,  257  ff. 

eech,  83  f .,  139  f.,  194- 
Light  intensity,  change  of,  135  ff. 

umax,  203. 
Limpets,  85. 

Jmulus,  90. 

Jneus,  147. 

uttorina,  205  f.,  209,  222  f. 

Lloyd  Morgan's  Canon,  31,  264,  287. 

l,obster,  194. 

locality  memory,  232  f. 

Locality  survey,  233  f. 

Localized    stimulus,    reaction    to,    59, 

173  ff- 
Lymnaea,  85. 

Macromysis,  207. 

Mayfly,  203,  266  f.,  312. 

Maze  method,  272  ff. 

Maze,  rotated,  282. 

Maze  running,  sensory  cues  in,  280  ff. 

Mechanical  stimulation,  effect  on  photot- 
ropism,  203  ff.,  207. 

Mechanism,  22. 

Metridium,  73  f .,  137,  254  f- 

Microstoma,  80. 

Migration,  of  fish,  in;  of  birds,  113. 

Mimicry,  230. 

Mind,  evidence  of,  27  ff. 

Mollusks,  chemical  sense,  84  ff.;  hear- 
ing, 117;  response  to  change  of  light 
intensity,  140;  color  vision,  148; 
geotropism,  185  f. ;  210;  size  percep- 
tion, 222  f. ;  adaptation,  252;  learn- 
ing, 257. 

Monkeys,  115,  169,  171,  224,  227  ff., 
261,  270  f.,  275,  290,  296  ff.,  299  f. 

Mosquito,  194,  252. 

Moth,  123. 

Mouse,  171,  238  f. 

Mouse,  dancing,  169  f.,  228,  275,  290, 
3ioff. 

Movable  sense  organ,  243  f . 

Movements,  variety  of,  condition  of 
memory  ideas,  304  f. 

Movement     sensations,     attention     to, 

305  f- 

Movement  vision,  216  f. 
Moving     stimulus,     response     to,     76, 

215  ff.,  317. 


380 


Index 


Multiple  Choice  Method,  298  ff. 
Myriapods,  225. 
My  sis,  1 19. 

Necturus,  238. 

Negative   reaction,   varied  to   repeated 
stimulus,  247  f. 

Objective  methods,  20  ff. 

Olfactory  cues  in  maze  running,  281. 

Ophiura,  248. 

Orang,  300. 

Orchestia,  201  f.,  204. 

Organic  sensation,  3,  62. 

Oriented  reactions,  176  ff. 

Orienting    flights,     106.    See    Locality 

survey. 
Otocysts,  116  f.    See  Statocysts. 

Pagurus,  89. 

Pain,  62,  83,  in. 

Palsemon,  88,  118,  188. 

Palsemonetes,  118,  202. 

Paramecium,  63  ff.,  174,  178  ff.,  211  f., 

262,  314. 
Patella,  85. 

Pawlow's  Method,  57  f.,  133,  171,  228. 
Pecten,  217,  222. 
Penaeus,  188. 
Periodical    changes    in    tropisms.    See 

Rhythms. 

Photokinesis,  143  ff.,  207. 
Phototropism,  193  ff. 
Physa,  85. 
Physiological  state,  effect  of,  60,  70  f., 

74,  80,  82,  86,  106,  265. 

Pig,  3°o- 

Pigeon,  165,  226,  269,  275,  281  f.,  312. 

Planarians.     See  Flatworms. 

Plants,  color  reactions,  158  f. 

Platyonichus,  188. 

Play,  305  f . 

Pleasantness,  45  f.,  70,  143  f.,  277. 

Porcupine,  271. 

Porthesia,  203. 

Preference  Method,  55,  92,  154,  146  f., 

149  f. 

Prepotent  reactions,  258,  260  f.,  316  ff. 
Prolonged  action  of  light,  201  f . 
Protozoa,  39  ff.,  chemical  sense,  63  ff. ; 

response  to  change  in  light  intensity, 

13=;;    color  vision,   146;    geotropism, 

178  ff. 


Punishment,  263  ff.,  277  f.^ 

Pupillar  reflex,  157. 

Purity  of  water,  effect  on  phototropism, 

203. 
Puzzle-box  experiments,  267  ff. 

Rabbits,  170  f.,  267,  292. 

Raccoon,  134,  169,  224,  261,  267,  271, 
293  ff.,  297  f. 

Ranatra,  201,  203,  315. 

Random  movements,  198  f. 

Rat,  132  f.,  171,  224,  239,  263,  269,  272, 
275,  280  f.,  290  f .,  300,  309  ff. 

Reaction  time,  as  evidence  of  discrimina- 
tion, 83. 

Recognition  of  visual  landmarks,  233  ff. 

Repeated  stimuli,  reaction  to,  246  ff., 
251  ff. 

Repetition,  ^Law  of,  257  f . 

Reptiles,  hearing,  131 ;  color  vision, 
163  f . ;  distance  perception,  140  f. 

Rest,  effect  on  phototropism,  206  f. 

Reward,  266  ff.,  277  f. 

Rheotropism,  178,  211  f. 

Rhythms,  acquired,  284  f. 

Righting  reaction,  178,  183,  188. 

Sagartia,  74,  137- 

Salamander,  163  f.,  238. 

Salivary  reflex,  57. 

Sarsia,  137  f. 

Sea-anemones,  71  ff.,  137,  182,  193,  251, 

254  f.,  256,  259. 
Sea-urchin,  175,  187,  252,  315. 
Semi-circular  canals,  190  ff. 
Sense-organs,  significance  of,  317. 
Sensibility  to  difference,  142  f. 
Serpula,  148. 

Sex,  effect  on  learning,  311. 
Sex  reactions,  89,  93  f . 
Simocephalus,  150. 
Size,  visual  perception  of,  222  ff. 
Skin  sensitiveness  to  light,  160,  163  f. 
Skioptic  reactions,  138  ff. 
Slug,  185,  203,  247. 
Snail,  84  ff.,  185  f.,  259. 
Space  perception,  176,  216,  242  ff. 
Sparrow,  268. 

Spatially  determined  reactions,  172  ff. 
Spider,  5  ff.,  89,  119  f.,  154,  221  f.,  237  f., 

242,  252,  258  f. 
Spirographis,  193. 
Squid,  209. 
Squirrel,  271,  312. 


Index 


Spathidium,  69. 

Starfish,  186  f.,  220,  248  ff. 

Statocysts,  117  ff.,  181,  183,  186,  188. 

Statoliths,  128. 

Stentor,  68,  247,  249,  251,  253. 

Stoiachactis,  74. 

Structure,  as  evidence  of  mind,    35  ff. ; 

as  evidence  of  discrimination,   54  f. 
Support,  sense  of,  239. 
Systems  of  successive  movements,  278  ff. 

Tactual  cues  in  maze  running,  281. 

Talbot-Plateau  Law,  152. 

Taming,  266. 

Tealia,  73. 

Telasthetic  taste,  81. 

Temora,  203. 

Temperature,    effect   on    phototropism, 

202. 
Temperature,    responses    to,    61,    214, 

249  f. 

Termites,  102,  155. 
Thigmotaxis,  66,  81. 
Thyone,  86. 
Tiaropsis,  138. 
Tones,  hearing  of,  133  f. 
Touch,  61  f.,  67  ff.,  75  ff.,  79  ff.,  87,  281. 
Toxopneustes,  141. 
Tropism,  193  ff. 


Tubularia,  71,  73,  137. 
Turtle,  165,  192,  224,  239,  274. 

Ultra-violet  rays,  147,  155,  252. 
Unpleasantness,  83,  89,  143  f.,  246,  250  1., 

264,  267,  277,  284. 
Useless   movements,    dropping   off,    258 

ff.,  288;  organization  into  systems,  283. 

Vanessa,  210. 

Variability  of  behavior,  as  evidence  of 

mind,  29  f. 
Vertebrates,     chemical    sense,    109    ff. ; 

hearing,   126  ff. ;   color  vision,  159  ff. ; 

geotropism,  190  ff. 
Virbius,  119. 

Visual  cues  in  maze  running,  281  f. 
Vision,  reaction  to  change  in  intensity, 

J35  ff-  >    photokinesis,   143  ff. ;    color 

vision,    144  ff. ;    of  motion,    216  ff. ; 

image   vision,    218   ff.;     of   distance, 

237  ff- 

Vitalism,  20. 
Volvox,  136. 
Vorticella,  251. 

Wasp,  91,  109,  156,  234  ff.,  286  f.,  291. 

Weber's  Law,  169. 

Wetness,  effect  on  phototropism,  206. 


INDEX  OF  NAMES 


The  numbers  refer  to  the  pages  on  which  the  work  of  the  writers  is  cited, 
whether  their  names  appear  in  the  text  or  not. 


292. 


i6o/ 


Abbott,  170,  240,  267,  278, 
Aderhold,  178. 
Allabach,  255. 
Allen,  281,  310. 
Andreae,  105. 
Andrews,  102,  155. 
Arkin,  83,  140,  198. 
Axenfeld,  210. 


Babak,  165. 

Baldwin,  306. 

Balss,  88. 

Bancroft,  199. 

Barber,  133. 

Bardeen,  79  ff.,  144. 

Bateson,  88,  no,  117,  142, 

Bauer,  161,  222. 

Baunacke,  186. 

Beer,  20  f.,  118,  188,  241. 

Bell,  88,  119,  217. 

Bentley,  160,  260,  262. 

Bernoulli,  128. 

Bert,  149  f. 

Bertkau,  90. 

Bethe,  17  f.,  20  ff.,  88,  96 

105  f.,  119,  124  f.,  189  f. 
Bigelow,  127,  191. 
Bingham,  226,  229. 
Bittner,  199. 
Blauuw,  195. 
Bogardus,  277  f. 
Bohn,  185,  193,  195,  205  ff.,  210,  222  f 

284. 

Bonnier,  106  f. 
Bouvier,  236. 
Boys,  120. 

Breed,  167,  224,  226. 
Breuer,  190. 
Brossa,  167. 
Bran,  102. 


ff.,  101,  103, 
,  231  f.,  263. 


Brundin,  204,  206  f. 
Buddenbrock,  186. 
Bunting,  188. 
Burford,  167. 
Burr,  112. 

Buttel-Reepen,  von,  105  ff.,   125,  158, 
232  f. 

Carpenter,  207. 

Carr,  280. 

Casteel,  224. 

Chidester,  88. 

Churchill,  273. 

Claparede,  17  f.,  309. 

Clark,  1 88. 

Coburn,  299. 

Cole,  L.  J.,  in,  223  f. 

Cole,  L.  W.,  168  f.,  224,  228,  261,  266  f., 

271,  293  ff. 
Colvin,  168. 
Conradi,  132. 
Copeland,  in. 
Cornetz,  96,  100. 
Cowles,  141  f.,  220. 
Craig,  132. 
Crozier,  199. 
Cyon,  113,  190. 


Dahl,  119,  237. 

Darwin,  8,  15  ff.,  81  f.,  104,  117,    140, 

3IS- 

Davenport,  152,  180,  185,  187. 
Dawson,  85,  186. 
Day,  262. 
Dearborn,  189. 
Delage,  119,  188. 
Dellinger,  40. 
DeMoll,  242. 
Descartes,  14  ff. 
De  Voss,  171. 

383 


Index 


Dice,  202. 

Dodson,  265  f. 

Drew,  117. 

Driesch,  20. 

Drzewina,  87,  206,  260,  284. 

Dubois,  140  f.,  163  f. 

Dufour,  155. 

Edinger,  309. 
Eigenmann,  160. 
Emery,  123. 
Engelmann,  117,  136. 
Enteman,  233,  291. 
Erhard,  150. 
Essenberg,  91,  202. 
Esterly,  189,  209. 
Ewald,  151  f. 

Fabre,  93  f.,  233,  235  f. 

Ferton,  236. 

Fielde,  102  ff.,  124,  252,  273 

Fiske,  306. 

Fleure,  73,  75,  137,  2SQ. 

Flourens,  190. 

Forel,  17,  19  f.,  91  ff.,  97,  99  f.,  103, 

124  f.,   155,   210,  225,  243. 

Frandsen,  185,  203,  247. 
Franz,  S.  I.,  320. 
Frisch,  von,  151,  156  f.,  161,  226. 
Frohlich,  186,  188,  191. 

Gamble,  184  f.,  201,  207,  209,  315. 

Ganson,    171. 

.Carrey,  213  f. 

Gee,  74,  139,  ig4,  284. 

Ghinst,  van  der,  182. 

Giltay,  104. 

Glaser,  248  f. 

Goltz,  100. 

Graber,  10,  55,  89,  92,  121  f.,  137 

154  f.,  160. 
Gregg,  296. 
Groom,  201. 
Groos,  A.  O.,  158. 
Groos,  K.,  305. 
Gurley,  213. 

Hatchet-Souplet,  237. 
Hadley,  194,  199. 
Haecker,  260. 
Haenel,  309. 
Haggerty,  297. 
Hahn,  134. 


105 


140 


Hamilton,  298  f.,  309. 

Hargitt,  137,  140,  193,  252,  256. 

Harper,  180  f.,  198. 

Haseman,  284. 

Henke,  277  f. 

Hensen,  118  f. 

Herms,  199. 

Herrick,  C.  J.,  259  f. 

Herwerden,  van,  152. 

Hess,  141,  148  ff.,  153,  IS6  f-j  l6o  Q 

1  66. 
Hesse,  136,  140,  144,  158  f,  218  f.,  242, 

Hinstedt,  164. 

Hofer,  129  f. 

Hoffmeister,  140,  315. 

Hoge,  263,  277. 

Holmes,  194,  198,  201  ff.,  205,  208,  252 

3iS  f. 

lubbert,  276,  311. 
iudson,  319  f. 
Hunter,    132  f.,   229,   275,   282,   290  f., 

293,  297  f.,  312. 

'anet,  123. 

Jennings,  40  ff.,  48  f  .,  57,  60,  64  ff.,  73  f., 
!35,  137,  174  f.,  186,  196  ff.,  200,  209 
ff.,  246  f.,  249,  251,  253  ff.,  314. 

ensen,  179. 

ohnson,  G.  R.,  199. 

ohnson,  H.  M.,  133,  226  ff.,  309. 

oubert,  142. 

Calischer,  133. 

Canda,  181,  185  f. 

Catz,  1  66. 

Ceeble,  184  f.,  201,  207,  209,  315. 

Kellogg,  92. 

Cepner,  42  f.,  80. 

Cienitz-Gerloff,  105. 

innaman,  115,  169,  224,  229,  271,  275, 
296. 
iine,  12. 

Cohlrausch,  167. 
Corner,  126. 
•Crall,  308. 


159. 

•Creidl,  126  f.,  188. 
ribs,  83. 

Ashley,  167  f.,  224. 
-ee,  126,  190  f. 
inert,  80. 


Index 


385 


Lewis,  152. 

Locke,  3. 

Loeb,  n,  17,  20,  22,  30  f.,  72,  75,  142  ff., 

146,  152,  155,  158,  182,  190,  194  ff., 

198,  201  ff.,  209. 
Lohner,  84. 
Lovell,  156. 
Lubbock,  10,  94  ff.,  100,  123  f.,  149  f., 

155  *•,  220,  227,  231. 
Lund,  68. 
Lyon,  179  ff.,  187,  202,  211  ff. 

McClendon,  43. 

McCook,  90. 

MacCurdy,  141. 

McGinnis,  210. 

Mclndoo,  93,  99,  102. 

McPheeters,  296. 

Maday,  von,  309. 

Marage,  126. 

Massart,  179. 

Mast,  43,  69,   136,  146,  162  f.,   196  f., 

249  f.,  298. 
Mendelssohn,  214. 
Merejowsky,  150. 
Metalnikow,  68. 
Meumann,  309. 
Mills,  ii  f.,  192,  270. 
Minkiewicz,  147  f.,  153. 
Mitsukuri,  205. 
Mobius,  263. 
Montaigne,  14  ff. 
Moody,  69. 
Moore,  A.,  181. 
Moore,  A.  R.,  181,  186,  203. 
Morgan,  Lloyd,  7,  n,  25  f.,  31,  81,  112, 

1 1 8,  289,  291. 
Morgulis,  171. 
Morse,  284. 
Muller,  G.  E.,  309. 
Miiller,  H.,  104. 
Munk,  133. 
Murbach,  117,  183. 

Nagel,  34,  72  ff.,  76  f.,  82,  84,  87  f., 
90  f.,  no,  117,  137,  140,  164,  183, 
215,  218,  252,  254  f.,  257. 

Norman,  83. 

Oelzelt-Newin,  57. 
Oltmanns,  197. 
Orbelli,  228. 
Osten,  von,  307. 
Ostwald,  202  f. 
2C 


Parker,  74,  82  f.,  89,  109  ff.,  124,  127  ff., 

I4O,    159  f.,    164,    189,    198,    2OI  f.,    2O5, 
207,  210,  223,  254  f.,  284. 

Parshley,  82. 

Pawlow,  57,  133,  171,  228. 

Pearl,  79  f.,  90,  174  f.,  184. 

Pearse,  71,  86,  137,  153,  164,  231. 

Peckham,  5  f.,  89,  109,  120,  154,  233  f., 

252,  259. 
Perkins,  185. 
Perris,  93,  109. 
Petrunkewitch,  221. 
Pfungst,  308. 
Pie"ron,  85,  96,  100  f.,  253  f.,  257,  259, 

284. 

Plateau,  157  f.,  210,  217,  223,  225,  237. 
Plessner,  142. 
Pollock,  72. 

Porter,  165,  226,  238,  268  f.,  275. 
Pouchet,  142,  199,  203. 
Prentiss,  119. 

Preyer,  10  f.,  86,  117,  247  f. 
Pritchett,  89!.,  120. 

Radl,  122,  180,  190,  194,  212,  214. 

Raspail,  112. 

Rawitz,  217,  252. 

Reese,  in. 

Regen,  122  f. 

Reighard,  161. 

Revesz,  166. 

Rhumbler,  39  f. 

Richardson,  238  f.,  312. 

Riley,  C.  F.  C.,  252. 

Riley,  C.  V.,  93- 

Risser,  112. 

Rockwell,  283. 

Romanes,  4,  8  ff.,  30  f.,  75,  113  f.,  117, 

137  f.,  141,  186  £.,  238. 
Root,  43- 
Rothmann,  133. 
Roubaud,  214,  317. 
Rouse,  165,  226,  269,  275,  281  f. 
Rousseau,  12. 
Royce,  36. 
Ryder,  140. 

Sackett,  271. 

Sanford,  309. 

Santschi,  99  f.,  220  f. 

Schaeffer,  41  ff.,  50,  68,  263. 

Scheuring,  242. 

Schmid,  146. 

Schwartz,  178. 


386 


Index 


Sewall,  190. 

Shelf ord,  in. 

Shepherd,  134. 

Sherrington,  216,  258,  303  f. 

Small,  192,  269,  272,  275,  281,  290  f.,  312 

Smith,  A.  C.,  Si  f. 

Smith,  E.  M.,  171. 

Smith,  S.,  262. 

Sosnowski,  181. 

Steiner,  190. 

Stevens,  153  f. 

Stocking,  263,  277. 

Strasburger,  199  f.,  202. 

Strong,  112  f. 

Swift,  133. 

Szymanski,  228,  264. 

Taliferro,  42  f.,  80. 

Thorndike,  8,   n  f.,   228,   261,   268  ff., 

273  ff.,  287,  289  f.,  292  f. 
Tiedemann,  141. 
Torrey,  34,  71,  182,  199. 
Tower,  122. 
Towle,  204. 
Trembley,  143. 
Triplett,  127,  263. 
Turner,   100,   123,   144,   156,   209,   221, 

225  f.,  233,  236,  264,  312. 

Uexkull,  von,  20  f.,  141,  175,  315. 
Ulrich,  311. 

Vaschide,  12. 

Verworn,  n,  117,  178  f.,  183,  192,  195. 

Vincent,  276,  281  f. 


Wager,  181. 

Wagner,  G.,  182,  247,  251. 

Walton,  A.  C.,  259. 

Washburn,  160,  170,  240,  260,  267,  278, 

292,  302  f. 
Wasmann,  19,  98  ff.,  101,  123  f.,   230, 

255- 
Watson,   20,   22  f.,   113,   163,   167,   171, 

237,  276  f.,  280  ff.,  309  f. 
Waugh,  171,  238  f. 
Weld,  124. 
Wenrich,  222. 
We"ry,  104  f.,  158. 
Weve,  159. 

Wheeler,  96,  123,  214. 
Whitman,  139,  238. 
Will,  91,  122. 
Willem,  217. 
Wilson,  143,  146. 
Wodsedalek,  203,  266  f.,  312. 
Wundt,  5  f. 

Yarb rough,  312. 

Yerkes,  R.  M.,  34  ff.,  60,  77,  130  f.,  138 
f.,  144,  150,  167,  171,  175,  183,  192, 
201  f.,  204,  214,  228,  239,  263  ff., 
273  ff.,  281,  290,  299  f.,  300,  310  ff. 

Yoakum,  271,  312. 

Young,  1 68. 

Yung,  84,  140  f.,  147. 

Zeliony,  133  f. 
Zenneck,  128, 


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